Andy Larsen: Will City Creek or Big Cottonwood Creek flood? An update on hydrologists’ forecasts

One fun aspect of this data column is the ability to bounce to whatever the topic du jour is — after all, there’s not much that can’t be looked at analytically to at least some degree.

In Utah at this moment, that topic is our melting snowpack — and whether or not it will result in flooding. And at every turn, I’ve been impressed at the wealth of information available from regional hydrologists and forecasters (tools I shared with you in our first flooding column a few weeks ago).

In this update, I have new information on the likelihood of City Creek flooding, along with an updated monthly water supply forecast that helps us nail down where we’ll see extraordinary snowmelt — and where we won’t. Finally, we’ll look again at the short-term flooding forecasts, in terms of what to expect over the next 10 days all over our state.

Let’s get started.

New monthly water supply forecast

This section isn’t a flooding forecast, to be exact, but instead a water supply forecast. The United States Department of Agriculture’s National Resources Conservation Service issues these forecasts that reflect how much total water it anticipates to flow through a certain stream over the course of months, in this case, from May to July. This information is used by farmers and ranchers, along with those who decide how much water to keep in Utah’s reservoirs vs. sending it further downstream. They issue these forecasts on the first of every month.

Here’s the May 1 forecast for the major creeks affecting Salt Lake County:

If you’d like, you can compare them to the April forecasts posted here. As you’d expect, we have a little bit more certainty in May than we had in April, so the ranges of projected outcomes are tighter.

We look even more poised for record-setting streamflows in Big Cottonwood Creek — we’ll dig into that in a second. In Little Cottonwood Creek, there’s about a 50/50 chance we set a new record in the total amount of water that will pass through — though reaching flood stage is currently not considered likely. Meanwhile, further north, we’ve seen enough to expect those water supply numbers to likely or certainly fall short of the record, though still stand multiples above what would happen in a normal year.

Looking elsewhere around the state:

• The USDA projects the inflow to Pineview Reservoir in Weber County will be about 460% of a normal year — which would tie the record set in 2011. It could be more.

• The median projection for the Colorado River station at the Glen Canyon Dam is 174% of a normal year, well below the record set of 272% in 1917. Farther east, the Colorado River station near the ghost town of Cisco, Utah projects water supply of 156% of a normal year, but well below the record of 261% set in 1984.

• It’s a similar story for the San Juan River near Bluff: a median projection of 174% of normal, but well off the record of 399%. Expect a good amount of water down in Southeastern Utah, but it won’t be the highest in history.

Big Cottonwood Creek

I want to zoom in on Big Cottonwood Creek, though, because of those expected record-setting flows referenced above.

First of all, here’s the chart of this season’s snowpack up at Brighton, at the head of the Big Cottonwood. The blue line is 2023′s snowpack levels, the green line is 2022, while the purple line is the median of the last 30 years of snowpack.

We have lost a significant amount of snowpack already in the last week of April and the first week of May — but there’s still much more snow left to go. Whether or not we see that all come down at once or more slowly will determine whether there’s flooding.

How likely is each outcome? Here’s the Colorado Basin River Forecast Center’s projections:

What we see is a nearly 50% chance of getting past the creek’s flood stage at 798 cubic feet per second. Furthermore, the CBRFC predicts a month-long wide window for when that peak could occur, anytime between May 22 and June 22.

There’s also a decent chance of more significant flooding: a 25% chance of reaching at least 872 cubic feet per second, and a 10% chance of reaching 1,092 cubic feet per second. That would surpass the historical peak of 925 cubic feet per second set back in 1984.

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

• At 798 cubic feet per second, minor flooding will occur to areas adjacent to Big Cottonwood Creek. The main areas affected will be those most susceptible to bank erosion at higher flows, and those located close to the creek. The flooding will threaten those areas from the outlet of Big Cottonwood Creek to the Jordan River.

• At 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 will receive flood damage.

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

So if you live along Big Cottonwood Creek, it’s probably prudent to watch the CBRFC’s forecast, at least from late May through most of June.

City Creek update

We’re only as good as the information we have.

I want to give a quick update on the story I wrote two weeks ago with regards to the likelihood of Utah’s City Creek flooding.

You see, at one point — the critical point in which my article was written, actually — the National Weather Service’s CBRFC projected nearly a 100% chance of City Creek flooding during this season, simply due to the amount of melting snow runoff expected. However, after my article was published, that percentage chance of flood flows dropped to about 40%.

What happened?

Well, the weather service updated its estimate of what level of stream flow would cause flooding.

In particular, the CBRFC had previously said that a flow of 103 cubic feet per second would cause flooding in areas of Salt Lake City at North Temple and around 400 West. That, forecasters said, was due to previous flooding that had occurred at that level of runoff.

But it turns out that that was an out-of-date estimate of flood flow. Since then, Salt Lake City and Salt Lake County have upgraded City Creek’s infrastructure so that it can contain way more water.

So now (and only updated after the article was written), the CBRFC’s new estimate of the level of flow needed to cause flooding is 210 cubic feet per second. It’s also worth noting that Salt Lake City Public Utilities officials think they’d likely even be fine above those levels. They say that they’d be able to divert some of those flows if need be, so even those much higher capacities may be an underestimate.

Still, even just the increase of flow from 103 cubic feet per second to 210 has a major impact on the likelihood of flooding. Here’s the most recent forecast chart indicating City Creek’s peak flow likelihood:

Imagine a horizontal line across this graph at 103 cubic feet per second — you can see that flows above that level are extremely likely. Now imagine one across the graph at 210 cubic feet per second — there’s probably about a 40% chance that happens. The previous story (in print, anyway) showed the older, inaccurate information; the online version was updated within hours, but still showed some older info for a while there.

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

“It’s unlikely we will see flows reach free flow critical thresholds this year,” Glen Merrill, senior hydrologist at the Salt Lake City weather forecast office told me, “unless we get a prolonged large rain on snow event during the peak melt period, or a much more significant heat wave that will melt both the remaining southerly facing snowpack in the upper canyon, and the more significant northerly facing snow which exists to lower elevations, at the same time.”

But in the end, flooding in City Creek looks less likely than I thought previously — not so much because circumstances have changed, but because how they’re reported to us in the public have.

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