2019/10/15

Thanksgiving Long Weekend 2019

How did the weather for the 2019 Thanksgiving Long Weekend compare to recent years?

Temperatures

Thanksgiving 2019 was a late Thanksgiving, and it wasn't really cold, but it wasn't warm either.

Saturday's High of 11.9°C was above the Thanksgiving average of 11°C, and Friday also broke 10°C at 10.3°C. Things cooled off from Saturday & Sunday at 7.2°C & 4.4°C respectively.

In recent years:

  • Thanksgiving Monday in 2018 was below freezing at -1.8°C
  • 2017 had two days above 15°C
  • 2016's Highs were all below 3°C
  • 2015 was a "hot" thanksgiving with one High at 26°C and another at 21°C


Precipitation

It was a fairly rainy Thanksgiving weekend, with the International getting 2.7mm on the Sunday and 3.6mm on the Monday. And Blatchford (not shown) was double that with 6.4mm of precipitation on the Sunday and 6.9mm on the Monday.

This year the International also recorded "trace" amounts of snow, but not enough to count as a measurement. Thanksgivings 2016-2018 were all snowier than that, with 2016 & 2018 getting a lot, and 2017 a little.

For 2015-2019 we've now had 5 Thanksgivings in-a-row with precipitation. And just before that from 2010-2014 we had 5 in-a-row without any.

2019/10/01

September 2019 Review / October 2019 Preview

Today we're going to be taking a look back at September of 2019. The summer of 2019 was generally pretty disappointing, and did that continue past labour day? Let's take a look.

High Temperatures


Here we have the daily High temperatures for September 2019 compared to the range of temperatures for the past 30 years.

The month has ended with a coldsnap, with the 27th-30th all around 5°C which is right at the bottom of our range for September.

In the middle of the month we had 10 days of above average temperatures. Most of those days were around the 75th percentile and weren't extremely warm, but 10 straight "warm" days was more than we had managed all summer.

The little bargraph at the bottom of this dashboard shows how September 2019's Highs compared to other years. And 2019 is a little bit hard to see, because it's just a touch below the 30-year average. It is dwarfed by 2018 which was -6.5°C below average, and had the 3rd coldest Highs since 1880.




In September 2019 we didn't have any days hit 25°C, while the September average is about 3 days. We did have 12 days hit 20°C though, compared to an average of 10.

We also had 2 Highs below 5°C. That is less than the 7(!) we had last year during the ridiculously cold September of 2018, but most Septembers have none.

The average High for September 2019 of 17°C is right in the middle of the pack, and was similar to 2014-2016. It was a lot warmer than 2018's 11.1°C, but a lot colder than 2009's 22.6°C.


Low Temperatures


For the Low temperatures September 2019 was mostly just above average...right up until the last week when things took a nosedive. We had a few days right at the bottom of the 30-year range, but that is still above the coldest-since-1880 records which are down around -10°C.

The little bargraph at the bottom again shows how the Lows compared. September 2019's average Low was 1.4°C above the 30-year average, and was almost 5°C above 2018.




This September Blatchford record 3 frosts. That is down from the 8 we had in 2019, but it is a lot compared to most Septembers. In recent years we went frost-free in September in 2015-2017 & 2011-2012.

The average Low of 7.3°C was slightly on the warmer side of things - well above 2018, but also above 2014-2016.


First Frost


The first fall frost of 2019 on September 27th was 3 days after Blatchford's average of September 24th. That made the frost-free season for 2019 145 days long, from May 5th to September 27th, and that is right around average.


2019 So Far


After the cool summer there had been some hope that September would finally turn things around. It didn't really manage that though, with our average High for May-September coming in at 19.7°C. That was 0.7°C cooler than the 30-year average, and ranks 98th out of 139.

This chart can also be flipped to the Lows temperatures. The May-September average Low of 9.0°C was just -0.1°C cooler than the 30-year average.



Combining the Highs & the Lows, September 2019's Mean temperature of 12.1°C was 1.4°C warmer than the 20th century average for September. That makes this 2019's largest orange circle (or warmest month compared to the 20th century average) since back in March.

This chart can also be flipped to show the Highs or the Lows. For the Highs September 2019 is a small orange circle, at 0.1°C warmer than the 20th century average. For the Lows it's a large~ish orange circle at 2.6°C above the 20th century.




This set of charts can be a bit cryptic, but on a day-by-day basis it compares how warm or cold the year has been. The top section is for Mean temperatures, and then that is separated into the Highs and Lows.

For 2019 so far our mean temperatures are just a bit above the 20th century average at 0.34°C. In recent years 2018 was 0.9°C above the 20th century average, 2017 was 1.6°C, 2016 was 2.9°C, and 2015 was 2.6°C. The last year that was close to the 20th century average was 2009 at 0.2°C difference, and the last below-average year was 1996 at -1.7°C colder.

And while 2019's mean is sitting right now at 0.34°C above the 20th century average, the Highs are at -0.1°C below while the Lows are at 0.8°C above. We still have October-December to go though, and the winter months can really change things.


Precipitation


Switching over to precipitation, both Blatchford and the International were a little below the September average of 35mm. The International had 25.5mm and Blatchford had 24.2mm.




But even though our amount of precipitation was below average, our number of days with precipitation was a little bit above September's average of 10 days.



For the year so far Blatchford's total precipitation is a little bit above the average at 407.7mm, and the International is right around average at 373.8mm.

Snow

No snow!

Well, that's not quite right. The International reported "trace" amounts of snow on the 27th, 28th & 30th but they weren't enough to measure, so we're not counting them.



This chart has our history of September snowfalls for Blatchford (1880-2005) and the International (1960+).

2018 stands out with bubble after bubble after bubble for a record snowy month. But we also had significant September snow in 2017 and 2014, and one small snowfall in 2015. So in the last 5 years September snow has been quite common, but before that from 1992-2014 only two Septembers - 1999 & 2004 - had any snow.

We will talk a lot more about snow once it starts falling, but for today let's just look in awe at Calgary's 34.4cm of this September (which was all crammed into 4 days):





October Temperatures


Here's a look at what October could have in store for us.

We've mostly seen the end of our 20°C days, with October averaging only 1 per month. And we'll probably start seeing some below-freezing Highs, with October averaging 2 days below 0°C.

For Lows the frosts will undoubtedly start to ramp up, but October averages about 16 Lows above freezing.

2019/08/31

August 2019 Review / September 2019 Preview

It's month-in-review time again, but this month is going to be a little bit different because we will be using interactive dashboards for everything, instead of the typical charts. The charts are sometimes a bit prettier, but the dashboards give access to a huge amount of extra information, so we are going to see how well they work.

As usual, we will start with...

High Temperatures

Here we have the High temperatures for August 2019. The 30-year average for each day is shown by the grey line, and the orange & blue areas in the background are the range of temperatures over that last 30-years. The outermost orange & blue lines are the records for the warmest & coldest Highs since 1880.

And this August we spent a lot of the month below average, with only a few warm days breaking above the 75th percentile.

The little bar graph at the bottom compares the average High each August, going back to 1880. 2019 was about 1.7°C below the 30-year average, and the coldest since 2010. That ranks 2019 as 109th out of 139. The Highs for the past few Augusts had all been above average, and were 2-3°C warmer than 2019.

Let's take a closer look at the numbers:


This table has a lot of stuff going on, but it shows the daily High temperature for each August going back to 2000. The colour of each cell represents how warm or cold the day was, but in August most days are above 15°C so everything ends up as a big red blur. On the chart if you switch from "Absolute Colours" to "Relative Colours" that makes it easier to spot all of the warm & cold days.

With an average High of 20.7°C, August 2019 was on the cool side of the things, in line with 2010, 2004 & 2000. The recent August with the coldest Highs was 2007, averaging a whole degree cooler than this year at 19.7°C.

This August we had 6 days at or above 25°C, while 2014-2018 all had 10-14. And for days which hit 20°C August 2019 had 17, while the last few years were up around 25 or 27.

August 2019 only had one High below 15°C, with the 30th at 13.7°C. That's fewer than 2018 which had 3 cold Highs, and 15 years ago 2002, 2004 & 2005 all had August Highs below 10°C.


Low Temperatures

Here we have our August Lows, which bounced around the average all month.

Normally I say that the Lows are a bit boring compared to the Highs, but the little bar graph at the bottom of this chart is actually really interesting.

August 2019's Lows were 0.3°C below the 30-year average, and about the same as last year.

What is interesting about the Lows is just how much they have changed over time. From the 1880s-1960s the Lows in August were consistently 2-4°C below the recent average. If you flip back to the chart for the Highs from earlier you'll see that they don't have the same pattern at all, and have mostly just bounced around during the past century.


August 2019's average Low of 10.9°C was (unsurprisingly) on the cool side, but was about the same as 2018, 2009, 2006, 2004, etc. The coldest recent August average Low was 2005 at 9.5°C.

We had 1 Low below 5°C, with the 27th at 4.7°C. Last year had 2 cold Lows, and there have been a few others scattered over the past 20 years.

If you flip this dashboard over to Relative Colours you can really see how things cool off towards the end of August as the colours shift towards blue.


Warm & Cold Months

When we combine the Highs & Lows, overall August 2019 was just a touch colder than the 20th century average for August at 0.2°C below the longterm average.

This dashboard can also be switched to show that the Highs were 1.1°C colder than the 20th century average, while the Lows were 1.3°C warmer (which is how we end up just above average).


Something to note is that we've had 4 months of pretty mediocre Highs, with May-August all below or right around the longterm average. Looking at the past few summers we had a lot of warm months with big, orange bubbles, but not this year. To find another 4-in-a-row cold summer we need to go back to 2010, and then to 2005, 2004 & 2000.


Going back to our first chart from today we can set the range to May-August, and the Highs for 2019 ranked 110th out of 139. But on average that was still a degree warmer than summers like 2010, 2005, 2004, 2000, 1999, 1996...


Hot Days

2019's cool summer resulted in an overall lack of hot days, which we can see in this dashboard.

2019's hot-day-season is nearing its end, and so far from May-August we had 20 days hit 25°C, and only 5 hit 28°C. That's pretty similar to years like 2009-2013, but way down from 2014-2018. The past few summers had all been really warm, with 35-54 25°C days, 9-21 28°C days.

So in general the summer of 2019 was a lot cooler than 2014-2018, but not as unusually cold as we might think.


Precipitation


After above-average rain in June & July things calmed down a bit in August, with the Edmonton International a bit above average at 69.8mm and Blatchford a bit below at 49.9mm.



Even though the amount of rain in August was about average, we were above average for the number of rainy days. The average for August is 12 days, and this year Blatchford had 18 days which recorded precipitation and the International had 20 days.


For the year so far Blatchford's total precipitation is above average, while the International is just below.


September Temperatures

I would prefer to forget that September 2018 ever existed, but here is a reminder of just how abnormally cold it was. The colours here are the "Relative" version to make all of those unrelentingly cold days stand out a bit more. More typically though we will have a week or even two of days around 20°C (although in 2019 nothing would surprise me).


For September Lows (in "Absolute" colours this time) we can see that the frosts in blue are fairly rare, but 2018 again was something that we will hopefully not see repeat this year.

And that brings us to the end of our first dashboard-based month-in-review. Hopefully it worked out, and we will play around with the format a bit in the future.

Labour Day Long Weekend 2019

Do we really want to talk about Labour Day Long Weekend weather? I guess that we might as well...

Temperatures

The average High temperature for the Labour Day weekend is actually only 19.4°C, so I guess that we shouldn't be too shocked to have sub-20°C days?

  • In 2018 the smokey August was just ending, and the Friday reached 20.4°C, but the Highs for the rest of the weekend were between 15-18°C.
  • 2017 was a warm Labour Day weekend, with the Saturday at 25.5°C, and the Friday and Monday both above 20°C.
  • And for the three years from 2014-2016 there wasn't a single High above 20°C, although 2014 came close. 2015 & 2016 both reached scorching Highs of 16.4°C
  • The warmest recent Labour Day weekend was 2013 with two days hitting 25°C, and the coldest was 2000 with no Highs hitting 15°C.


Precipitation
(the title of this chart is wrong)

And precipitation is fairly typical.
  • 2018 had rain on the Friday, Saturday & Monday.
  • 2017 had rain on the Saturday & Sunday.
  • 2016 had rain on the Friday, and the International even recorded a bit of snow on the Saturday.
  • 2015 had rain on all four days for a total of 34mm.

Going back to 1995 the only Labour Day weekends with no precipitation were 1995, 1998, 1999, 2006 and 2010.


Updated for 2019:

And so Labour Day 2019 ended up with one day above 20°C, with the holiday Monday hitting 21°C. The Friday High was down at 13.7°C, which is cool, but 2015 & 2016 had much cooler days. And the Saturday and Sunday were just above 15°C. So across the weekend the temperatures were pretty close 2018, well below 2017, but above 2015& 2016.


For temperatures the International recorded rain on all 4 days, with Saturday having the most at 6.1mm. For Blatchford (which isn't shown here) the Friday had the most precipitation with 5.3mm.

2019/08/11

ABC's of Air Quality

Today we are going to talk about Air Quality.

We will start with a caveat: I am not an expert on this. If any experts read this they will think "This person is not an expert" and they will be correct, because I am not an expert.

But at the end of May 2019 Edmonton had a very, very, very, very, very, very, bad day for Air Quality.



During that very bad day Environment Canada and the media were focused on real-time reporting, because the public needed to be warned about the health risks. But hearing things like "On a scale of 1 to 10 this is a 9...now it's 12...now it's 25...now it's 72" left me with a lot of questions.
  • How often does it happen?
  • How often does it happen in May?
  • How does it compare to August 2018?
  • How bad is it?
  • Has it ever been this bad before?

I was unable to easily find answers to those questions, and so this is the Edmonton Weather Nerdery attempt at providing some context. All of the data today is from the province of Alberta, with the realtime data available at http://airquality.alberta.ca/map/ and the historic data from http://airdata.alberta.ca/.

Normally on this blog we talk about things which are pretty straightforward and objective like temperature: 0°C = 32°F = 273.15K. Air quality measurements are a bit different because they are an attempt to quantify the health effects that poor air has on people. Today we will be looking at two different scales - AQHI and AQI - and those numbers won't be a perfect match, and that is okay.

Let's jump right in with a dashboard that has a lot of things going on:



This dashboard is interactive, and as you hover your mouse over it (sorry, mobile users!) the data will update for different days going back to 1998.

Some Background

Stations:
  • Edmonton has 7 stations recording hourly air quality.
    • The Northwest station has data for 1998-2004; Central & East began in 2001; and the other stations began after that.
    • Some stations only record FPM2.5 (which we will explain in a second) while others also record Ozone and Nitrogen Dioxide.
  • The map in the middle of the dashboard shows the highest hourly FPM2.5 for each station on a given day.

FPM2.5:
  • FPM2.5 is the measurement of the mass of Fine Particulate Matter (2.5 micron) in the air in µg/m³. When the air is smokey that level of FPM2.5 goes up.
  • The chart at the top of the dashboard shows the peak daily FPM2.5 averaged for all stations.
  • The red dots highlight days with a peak FPM2.5 averaged for all stations of more than 200µg/m³. Typically we are around 20µg/m³, and so the red dots are 11 of the very worst days for Air Quality in the Edmonton region in the past 2 decades.

AQHI:
  • The Air Quality Health Index (AHQI) was created by Environment Canada and is only used in Canada. 
  • It is calculated from the levels of FPM2.5, Ozone & Nitrogen Dioxide. (Alberta uses a slightly modified version of the AQHI but to track other compounds, but to keep things simple we will be sticking with the Environment Canada Standard)
  • At the bottom left of the dashboard is the highest 3-hour average AQHI for each station on a given day.
  • AQHI has a scale of 1-10+.
  • From Wikipedia:


AQI:
  • The Air Quality Index (AQI) is at the bottom right of the dashboard. 
  • AQI is used internationally.
  • Separate AQIs can be calculated for individual compounds (Ozone, Nitrogen Dioxide, Sulfur Dioxide...) but we will only be using the level of FPM2.5.
  • At the bottom right of the dashboard is the 24-hour average AQI for each station on a given day.
  • AQI has a scale of 1-500.
  • From Wikipedia:

One thing to point out is that all of these are measured hourly, but AQHI is calculated as a rolling 3-Hour average, while AQI is a daily average. So in the charts today we will have a mix of peak daily FPM2.5 (because I think the peak is interesting), peak 3-hour average AQHI, and 24-hour average AQI.

With that background out of the way, lets take another look at the data:

May 30, 2019



So how bad was the air quality on May 30, 2019? Unfortunately we don't have a lot of history to compare it to because the data only goes back to 1998, but based on what we do have May 30, 2019 was off the charts.

  • The average of the peak FPM2.5 for all of the stations was 1,338µg/m³. Since 1998 the previous record had been August 19, 2010 way down at 433µg/m³.

  • The peaks for the individual stations ranged from Woodcroft at 450µg/m³; to Central at 867µg/m³; to East & St. Albert above 1,700µg/m³; and South was at 1,880µg/m³. The previous highest measurement at any station had been 450µg/m³ at the Central station on August 19, 2010.

  • The highest 3-Hour average AQHI (on a scale of 1-10+) ranged from Woodcroft at 24, Central at 46, South at 108, East at 114, and St. Albert at 132. The previous highest AQHI since 1998 had been 23 on August 19, 2010 again.

  • The AQI for the day (on a scale of 0-500) ranged from Woodcroft at 207 (Very Unhealthy) to East at 431 (Hazardous). The previous highest AQI since 1998 had been 222, also on August 19, 2010.

At the time it seemed like May 30, 2019 was extra-bad, and the data backs that up:
  • a typical day is around 20µg/m³.
  • a really, really bad day (10 in the past 20 years) hits 200µg/m³.
  • the previous worst-of-the-worst days (2 in the past 20 years) hit 400µg/m³.
  • and May 30, 2019 blew them all away with readings of 850, 1,400, 1,880, etc.

A Brief History



This dashboard shows the highest FPM2.5 and the median FPM2.5 for each year for all of the stations. With this we can see how things have changed on a yearly basis.

A few cautions about this:
  • This uses the yearly median FPM2.5 because the yearly mean (what we typically think of as an average) is too susceptible to extreme values. If we have 1 day at 1,880µg/m³ that is the equivalent of 90 "typical" days, and it drags the yearly mean way, way up. The yearly median is a better representation of a "typical" day throughout the year.
  • The red line is the average of all the stations, but this is affected by which stations have available data:
    • For 1998-1999 only the Northwest has data available, and its yearly mean tends to be higher than the other stations. So the "Average of All Stations" looks really high for 1998-1999, but that's because it is just the one station. 
    • For 2006-2019 the McIntyre station starts reporting, and its yearly mean also tends to be higher than the other stations, and so that also drags the overall average up.

20 years isn't really enough time to identify long-term trends, but with what we have available we can see:
  • A low yearly peak is around 100-150µg/m³, but 2002, 2010, 2013, 2014 & 2016-2019 all had at least one station with a peak of 300µg/m³. There have been more high-peaks in recent years, but that might just be because there are more stations reporting.
  • For the yearly median most stations are between 12-20µg/m³. But for both Northwest and McIntyre the median is often 5µg/m³ more than the other stations.
  • 2010 is a standout year, with all of the stations having a peak between 400-450µg/m³, and the yearly mean for all of the stations was also higher than normal, ranging from 20-25µg/m³.
Alberta's policy guidelines for particulate matter are here.

The 24-hour average target is 29µg/m³, and today we're mostly focusing on peak-numbers so we will maybe take a look at that at some point in the future. But for peak-numbers, Alberta's 1-hour target is 80µg/m³, and in the next section we will look at how often we high days.


High FPM2.5 Days Each Year



Here we are looking at the number of days with high FPM2.5 each year. The default for this dashboard is 200µg/m³ because that is a pretty bad day, but it can be adjusted to show 50µg/m³, or 100µg/m³, or 400µg/m³, or Alberta's target of 80µg/m³.

We've previously seen that 2010 & 2019 each had a really bad day, with FPM2.5 hitting more than 400µg/m³ and 800µg/m³ respectively. Those really bad days show up in the top part of this chart.

In the lower part of the chart we can also see that 2018 had a bunch of pretty bad days. The various stations recorded 3-5 days with FPM2.5 from 200-400µg/m³. That isn't as high as the peaks of 2010 & 2019, but in 2018 there were more days. If we lower the cutoff to 100µg/m³ (which is still pretty bad) 2018 had 6-8 days depending on the station, while 2010 had 4-7 and 2019 had 2.

These photos are from August 15th (average FPM2.5 of 187µg/m³), 16th (112µg/m³) & 17th (229µg/m³) 2018:


August 10th & 18th 2018 were both the worst of that week at 287µg/m³ and 293µg/m³, but I don't have any photos for those days. For most of the days that week the AQHI peaked from 8-14, and the daily AQIs were around 160-190. And in August 2018 things first got bad on August 7th with FPM2.5 of 74µg/m³, and they stayed pretty high until about August 26th.

So 2019 had one really, really bad day. 2010 had one really bad day, and a few more pretty bad days. And 2018 had a week of pretty bad days, and August was pretty rough overall.


FPM2.5 Throughout the Year



When our air turned to soup on May 30, 2019 a lot of people were saying "It's never this bad in May." We've already seen that May 30, 2019 was undeniably the worst-of-the-worst-of-the-worst, but this chart shows when throughout the year we get other pretty bad air quality days.

In the past 20 years our pretty bad air quality days have mostly been split between May and August:
  • For May
    • May 30, 2019 was terrible.
    • The Ft. McMurray wildfire was at the very beginning of May 2016, and Edmonton's air quality was generally fine, except for a single 200µg/m³ day on May 19th.
    • 1998, 2000, 2001 & 2002 all had high FPM2.5 days in May.
  • For August
    • August 2018 had a whole bunch of pretty bad days.
    • August 19, 2010 was a very bad day.

So it is true that in recent years August is the month for bad air quality, but going back a little bit further May has a history of it too.


Summary

Today we have mostly focused on the amount of 2.5micron Particulate Matter in the air. FPM2.5 is important because it is used to calculate the AQHI and AQI which are reported whenever the air quality is poor.
  • May 30, 2019 was Edmonton's worst air-quality day since 1998 by far. August 19, 2010 was the 2nd worst, but it wasn't even close. It is unfortunate that the data doesn't go further back than 1998, though.
  • August 2018 was also notable, with about a week of pretty bad days.
  • Recently August has had most of our bad days, but around 2000 there were also a number of them in May.
  • Over the past 20 years our "typical" FPM2.5 throughout the year has stayed fairly consistent at around 20µg/m³. And the worst days each year typically peak at 150-400µg/m³...until May 30, 2019 averaging 1,338µg/m³ for all the stations.


This final dashboard lets you filter by all the unhealthy/high/hazardous days at the various stations, using either AQHI or AQI. The ratings aren't a 1-to-1 match: in particular AQI puts a lot of days in the moderate category that AQHI ranks as low. But for the days with poor air quality the two scales have a lot in common.

Hopefully this all gives some context to Edmonton's recent history of Air Quality. The next time that the smoke rolls in it is important to remember the difference between the AQHI (Canadian, with a range of 1-10+) and the AQI (International, with a range of 0-500), and that both of them are mostly calculated using the FPM2.5.

2019/08/05

The North Saskatchewan River - from Top to Bottom

Today we are going to take a deep dive into the North Saskatchewan river in Edmonton. We will be using a whole series of interactive dashboards, and if you have ever had any questions about the river you might be able to find the answers here.


History: 1911-Present



To get things started, here we have daily data recorded at the Low Level bridge.
  • Stream Flow (m3/s) recordings in green go all the way back to 1911.
  • Depth (m) in blue starts in 1999.

The river level and flow fluctuate throughout the year, and the tall green and blue spikes represent floods and other high flow conditions.

Edmonton's largest recorded flood was June 29, 1915 with the water rising to 13.73m and 4,640m3/s:

From: https://citymuseumedmonton.ca/2015/06/29/june-29-1915-edmontons-river-valley-floods/

The photo of the Low Level Bridge with a freight train being used as ballast is quite well known, but the second photo of the Dawson bridge shows just how far the river spilled over its banks.

For some context, the river's average depth in the summer is around 3.5m with a stream flow of 300m3/s. During the great flood of 1915 the river's height was 10m (or 30'/3 storeys) above average, and the volume of water was about 15x more than average.

The second largest recorded flood was just over 30 years ago in 1986. The stream flow peaked at 3,990m3/s, and the depth was about 12m although I couldn't find an exact number for that. There are also reports of a large flood in August 1899 which reached 12.61m, but the records for that aren't complete. Other large floods were June 25, 1952 at 3,540m3/s and 10.59m, and 1944, 1954 and 1972 were also all around the 10m mark.

In recent years on June 23, 2013 the river reached 2,847m3/s and 9.25m. That was the same week that Calgary had its large flood. 2005 was another big year reaching 2,612m3/s and 8.85m.

This is what the 2013 mini-flood looked like:

 


Changes Throughout the Year



Here we have all of that daily data rolled into the calendar year from January-December.

Two caveats for this chart:

  • the depth data is only available for 1999 onward, so if you click through the various years the upper chart will be blank for 1911-1998.
  • I'm not really sure how accurate the readings are during the winter - for recent years since 2015 the winter stream flows aren't even measured.

With that said the river fluctuates throughout the year and has a fairly reliable peak in late-June or early-July. All of the largest floods occurred in June & July, but some years - 2003, 2007 & 2018 - also recorded big spikes in April & May during spring runoff.

One other thing to look for here is 2017, which is the year that the "accidental beach" hit the news. That summer the river depth was below 3.5m starting in early August, and that is low but it isn't really low.


3.5m is around the middle of the range for late-summer, but in 2018 the depth stayed closer to 4m through the end of the summer, and for 2019 so far we have generally been over 4.5m.

Beach to the left of me, Cloverdale to the right.
The beach was still around in 2018 & 2019, but the higher water levels create a wide (but not particularly deep) channel between it and Cloverdale which makes it hard for people to get to.


Changes Over Time



Are there any trends over the past century? That's a complicated question, because in 1963 the Brazeau Dam was constructed, and then in 1972 the Big Horn Dam was added. Those dams change the river's behavior throughout the year.

People often think that dams will stop floods, but once the reservoir behind a dam is full there's nothing more it can do. And so Edmonton's second largest recorded flood was in 1986, after both of the dams were built.

When we look at the peak stream flow each year it has averaged 1,100m3/s since 1911, and that average hasn't really changed since the construction of the dams. We have had a few less spikes though, because in the 61 years prior to the dams being built there were 6 years where the river peaked above 2,500m3/s, and in the 47 years since only 3 years have reached that high.

For the average flow during the summer, following the construction of the dams the average flow has been around 300m3/s, with some years just below 200m3/s while others have been as high as 400m3/s. Overall that is down compared to before the dams when the average was closer to 350m3/s, and the individual years typically ranged from 250m3/s to 450m3/s.

The most noticeable change here is on the low-end, with the river currently bottoming-out on average at around 100m3/s each year. Prior to the construction of the dams the average-minimum was half of that, at around 50m3/s.

Taproot Edmonton recently did a discussion of the history of the dams in: Dam Complicated. It high-lighted the importance of the dams for maintaining that minimum flow throughout the year - to provide a reliable supply for drinking water, agriculture, and industry (and also power generation).


Extreme Days



This dashboard lets us compare the number of high or low days each year, and we will start with the high days and flows of at least 1,000m3/s.

1,000m3/s isn't a flood, but it is on the high end of the river's typical range. And here we can see that the number of 1,000+m3/s has become much less variable since the construction of the dams. Since 1972 the high years have had at most 8-10 days with 1,000m3/s per year, while prior to the dams some years had 15 or even 25 days.



Flipping things around, here we have the number of low stream flow days each year. Since the construction of the dams the number of days with stream flow below 100m3/s has plummeted from an average of more than 150 days per year a century ago to less than 50 days today.

So the dams have moderated the high-flow days a bit, and have greatly reduced the number of low-flow days.


The Floods



And finally, in this dashboard we can compare how quickly the streamflow changed in the days immediately before and after Edmonton's historic floods. The first chart shows the streamflow each day, and the second shows the day-over-day difference.

Our most recent big flood in 2013 ramped-up from 400m3/s to 2,847m3/s over the course of 3 days. And then by 4 after the peak things were back down to around 1,000m3/s.

The great flood of 1915 took 2 days to go from 700m3/s to 4,470m3/s, and then on day 3 it increased a little bit more to 4,640m3/s. Then within 4 days it was back down to around 1,000m3/s. For a bit of trivia: during Calgary's big 2013 flood the Bow River peaked at 1,840m3/s and the Elbow peaked at 700m3/s, for a combined total of about 2,540m3/s.

Comparing all of these curves the ramp-up typically takes 2~3 days to go from average~ish to peak, with volumes that are 8x, 10x or even 15x above average. And then things ramp down a bit more slowly settling at around 1,000m3/s after a week.

And that's it for our big look at the North Saskatchewan River in Edmonton. I hope that this will be one of the more comprehensive discussions of the river that you will ever find, but if I've missed anything please let me know in the comments, or send me a message on twitter at https://twitter.com/yegwxnerdery.