Understanding Hangman (Latah) Creek’s Peak Flows

Understanding Hangman (Latah) Creek’s Peak Flows

If you’ve been near the confluence of Hangman (Latah) Creek and the Spokane River this week, you’ve seen it—the river running thick with brown sediment, turning the water into a muddy plume that stretches for miles downstream. This dramatic transformation comes after a major rain-on-snow event, where warm rain rapidly melted accumulated snow, saturating the soil and flushing huge amounts of sediment into the creek. These events aren’t new—but how common are they, and how have they changed over time? To answer that, we looked at historical hydrology data from 1948 to 2023 to see how this year’s event compares to past peak flows. 

What Is a Rain-on-Snow Event?

Rain-on-snow events occur when rainfall lands on an existing snowpack, causing rapid melting and an increase in runoff. The effect is often worse when the ground beneath the snow is frozen. In these conditions, instead of soaking into the soil, the meltwater and rain glide over the hard surface, gathering sediment, pollutants, and debris as they rush toward the nearest waterway. Hangman (Latah) Creek’s steep, erosion-prone landscape, combined with exposed agricultural soils, makes it particularly vulnerable during these events.

When the ground is unfrozen, some of the water can infiltrate the soil, reducing the speed and volume of runoff. But when frozen ground is covered by rain and melting snow, there’s nowhere for the water to go but downhill, carving into fields and streambanks and carrying massive amounts of sediment into the creek—and ultimately, the Spokane River.

This February, soil temperatures in the upper reaches of Hangman (Latah) Creek remained consistently below freezing, particularly during the peak storm period. Data from Tekoa, WA, shows that for much of early February, soil temperatures hovered between 26°F and 29°F, with a notable drop to as low as 17°F around mid-month. Compared to the 5-year and 10-year averages, which have seen temperatures inch above freezing during this time of year, 2025’s persistent frozen ground prevented infiltration, intensifying runoff volumes. As rain fell on top of the snowpack, there was nowhere for the water to go but downhill—taking tons of sediment with it.

This graph shows the average daily soil temperatures in Tekoa, Washington. This data comes from: https://www.greencastonline.com/tools/soil-temperature.

Patterns in Peak Flows: Then and Now

Historical records from 1948 to 2023 show that Hangman Creek’s largest flows consistently occur from late January through early March—the heart of rain-on-snow season. This week’s sediment-laden flow peaked at around 14,600 cfs, placing it well above the median annual peak flow of 6,090 cfs and higher than the average peak of 6,740 cfs. 

These events are now further complicated by changing climate patterns. Climate change is shifting flow patterns in the Spokane River basin, altering timing of snowmelt and our peak flows. While late-winter floods remain dominant, increasing mid-season temperature swings could make rain-on-snow floods like this year’s more frequent and unpredictable.

Though not record-breaking, this week’s flow is among the largest in the last couple decades and a reminder that the creek remains highly responsive to rain-on-snow conditions. Its timing in mid-February aligns with historical trends, but colder-than-average soils and persistent snowpack intensified the flood’s magnitude. The resulting surge sent alarming volumes of sediment downstream, affecting water quality as far as downtown Spokane.

Peak flow events, on average, have become less intense in recent decades compared to the mid-20th century. Since the late 1970s, large peak flow events have become less common. While occasional spikes still occur, such as in 2017 (11,500 cfs), recent years have seen lower peak flows and shifts in runoff timing. Notably, 2022’s peak flow occurred in mid-June—a sign of warming winters and later snowmelt. 

This graph shows the peak flows in Hangman (Latah) Creek each year since 1948, along with the moving 5-year average.

These shifting flow patterns highlight the need to prepare for more frequent, short-duration storms with high sediment loads. Warmer winters, shifts in precipitation timing, and land-use changes all play roles in how often and how severely the creek floods. With sediment from these events impacting water quality in the Spokane River, monitoring hydrology isn’t just about numbers—it’s about protecting the entire watershed.

Why It Matters

High flows aren’t just about swollen rivers—they have lasting impacts. Sediment pollution smothers fish habitats, disrupts aquatic ecosystems, and carries harmful pollutants downstream. With climate change expected to increase the frequency of rain-on-snow events, understanding the relationship between frozen soils, timing, and runoff is essential. By recognizing when and why these sediment surges occur, we can prioritize solutions like restoring riparian buffers, improving land use practices, and better preparing for future floods to protect both Hangman (Latah) Creek and the Spokane River.