Big Cottonwood Creek has significant flooding potential

One interesting aspect of this data column is the ability to dig into the topic of the day – after all, there’s not much that can’t be at least analyzed in some way.

Utah is all about our melting snowpack right now – and whether or not it will lead to flooding. And at every turn, I’ve been amazed by the wealth of information that regional hydrologists and forecasters (tools I shared with you a few weeks ago in our first column on flooding) are making available.

In this update, I have new information on the likelihood of City Creek flooding, as well as an updated monthly water supply forecast to help us pinpoint where we will and won’t see exceptional snowmelt. Finally, let’s take another look at the short-term flood forecasts for what to expect across our state over the next 10 days.

Let’s begin.

New monthly water supply forecast

More specifically, this section is not a flood forecast, but a water supply forecast. The US Department of Agriculture’s National Resources Conservation Service issues these forecasts, which reflect how much total water is expected to flow through a given stream over the course of months, in this case May through July. This information is used by farmers and ranchers, as well as those who decide how much water to store in Utah’s reservoirs rather than channeling it further downstream. They publish these forecasts on the first of each month.

Here is the May 1 forecast for the major streams affecting Salt Lake County:

If you want, you can compare them to the April forecasts posted here. As might be expected, we have a little more certainty in May than in April, so the range of forecast results is narrower.

We’re even better prepared for record-breaking water runs at Big Cottonwood Creek—we’ll get to that in a moment. At Little Cottonwood Creek, there’s about a 50/50 chance that we’ll set a new record for the total amount of water that will flow through – although reaching flood stage is currently considered unlikely. Meanwhile, farther north we’ve seen enough to believe that water supply numbers will likely or certainly fall short of record, although they’re still many times what would happen in a normal year.

Search elsewhere in the state:

• The USDA forecasts that inflow into Weber County’s Pineview Reservoir will be approximately 460% of a normal year – matching the 2011 record. It could be more.

• The mean forecast for the Colorado River station at Glen Canyon Dam is 174% of a normal year, well below the 1917 record of 272%. Further east, the Colorado River station near the ghost town of Cisco, forecasts Utah’s water supply is 156% of a normal year, but is well below the 1984 record of 261%.

• Similarly, the San Juan River near Bluff has a median forecast of 174% of normal, but well below the record 399%. A significant amount of water is expected in southeastern Utah, but it won’t be the highest on record.

Great Cottonwood Creek

However, due to the expected record-breaking flows referenced above, I would like to enlarge Big Cottonwood Creek.

First, here is the map of this year’s snow cover in Brighton, at the top of Big Cottonwood. The blue line represents snow cover in 2023, the green line represents 2022, while the purple line represents the mean snow cover over the past 30 years.

We’ve already lost a significant amount of snow in the last week of April and the first week of May – but there’s still a lot more snow ahead of us. Whether we see everything falling at once or more slowly will determine whether flooding occurs.

How likely is each outcome? Here are the forecasts from the Colorado Basin River Forecast Center:

What we are seeing is a nearly 50 percent chance of the creek passing the flood stage at 798 cubic feet per second. Additionally, the CBRFC is forecasting a one-month window in which this high could occur anytime between May 22nd and June 22nd.

There’s also a good chance for larger floods: a 25 percent chance of reaching at least 872 cubic feet per second and a 10 percent chance of reaching 1,092 cubic feet per second. That would surpass the all-time high of 925 cubic feet per second set in 1984.

What would each level mean? From the National Weather Service:

• At 798 cubic feet per second, there will be minor flooding in areas adjacent to Big Cottonwood Creek. The areas most affected are those that are most susceptible to bank erosion in the event of higher discharges and are located near the creek. The floods will threaten areas from the mouth of Big Cottonwood Creek to the Jordan River.

• At a rate of 861 cubic feet per second, moderate flooding will occur in Murray Park and downstream areas of Big Cottonwood Creek. Many low-lying residential areas along the river are damaged by flooding.

• And at 893 cubic feet per second, major flooding will occur along Big Cottonwood Creek from the mouth of Big Cottonwood Canyon to the Jordan River.

So if you live on Big Cottonwood Creek, it’s probably wise to keep an eye on the CBRFC’s forecasts for at least late May through most of June.

City Creek update

We are only as good as the information we have.

I want to give a quick update on the story I wrote two weeks ago regarding the likelihood of a flood in Utah’s City Creek.

You see, at one point — the juncture of writing my article — the National Weather Service’s CBRFC projected an almost 100 percent chance of City Creek flooding this season simply because of the expected amount of snow melt. However, after my article was published, that percentage chance of flooding dropped to about 40%.

What happened?

Well, the weather service has updated its estimate of how high the creek flow would cause flooding.

In particular, the CBRFC had previously stated that a flow of 103 cubic feet per second would cause flooding in areas of Salt Lake City near North Temple and about 400 West. According to forecasters, this was due to previous flooding that had occurred at that level of discharge.

However, it turned out that this was an outdated estimate of flood discharge. Since then, Salt Lake City and Salt Lake County have improved City Creek’s infrastructure so that it can hold much more water.

Now (and only updated after the article was written), the CBRFC’s new estimate for the amount of flow required to trigger a flood is 210 cubic feet per second. It’s also worth noting that Salt Lake City public utility officials think they could probably get by even above these levels. They say they would be able to divert some of those flows if needed, so even those much higher capacities may be an underestimate.

Still, just increasing flow from 103 cubic feet per second to 210 cubic feet per second has a major impact on the likelihood of flooding. Here is the most recent forecast chart showing City Creek’s maximum discharge probability:

Visualize a horizontal line across this chart at 103 cubic feet per second – you can see that currents above this level are extremely likely. Now imagine one on the other side of the chart traveling at 210 cubic feet per second – there’s about a 40% chance of that happening. The previous story (at least in print) contained the older, inaccurate information; The online version was updated within a few hours but still showed some older information there for a while.

Of course, the prognosis can change. What would have to happen for City Creek to flood this spring?

“Discharges are unlikely to reach critical free-flow thresholds this year,” Glen Merrill, senior hydrologist at the Salt Lake City Weather Forecasting Office, told me, “unless there is a sustained large rainstorm during the main meltdown period.” on-snow event, or.” A much larger heat wave that will simultaneously melt both the remaining south-facing snowpack in the upper canyon and the larger north-facing snow present at lower elevations.”

But in the end, City Creek flooding seems less likely than I previously thought — not so much because circumstances have changed, but because the way they are reported to us in public has changed.

Andy Larsen is a data columnist for The Salt Lake Tribune. You can reach him at alarsen@sltrib.com.