Donate

Snow to Streamflow: A Story of Decline [Part 1]

Fish Icon
Home » Blog » Snow to Streamflow: A Story of Decline [Part 1]
View of the mountains

Henry’s Lake mountains, photo credit Sarah Newcomb

Due to warmer growing seasons and denser forests, Henry’s Fork natural streamflow is declining and becoming less predictable.

Over the past 20 years, the Henry’s Fork has seen a decline in natural flow, or the amount of water that would flow down the river in the absence of any reservoirs or irrigation diversions (aka the water Mother Nature gives us). At the same time, we are starting to see rainier, less snowy years , meaning the total amount of water entering the watershed each year hasn’t changed, but less is falling as snow. So, how do we have the same amount of water coming in… but less becoming streamflow?

Natural flow graph
Rain graph for Henry's Fork Watershed

Where is the water going?

With funding from the National Science Foundation in 2023, Dr. Sarah Newcomb set out to answer this question as part of her dissertation research at Idaho State University. Sarah worked alongside the HFF Science and Technology Team to investigate how snowmelt and rain becomes streamflow in the Henry’s Fork. 

As a mountain hydrologist, Sarah studies how water moves through mountain systems. Water in the Henry’s Fork primarily starts as snow that falls high in three major tributaries: the upper Henry’s Fork, Fall River, and the Teton River. When rain falls and snow melts, the water enters the ground, saturates soils, recharges groundwater, and provides forests and plants with water. These underground processes can be hard to observe, but determine how much water is “leftover” for streamflow. This means that changes in streamflow indicate changes in these underground processes.

In this blog, we will dive into the processes contributing to the declining water supply. If you are interested in how we can characterize this change in predictive models, check out Part 2.

Part 1: Why is natural water supply declining?

Understanding a changing water supply requires considering a variety of climate and forest cover factors. In the western United States, some commonly cited reasons for streamflow decline include:

  • declining snowpack

  • declining total annual precipitation

  • less precipitation falling as snow

  • slower snowmelt

  • warmer temperatures

  • changes in forest cover

To understand the changes the Henry’s Fork is experiencing, we investigated these potential drivers using both on-the ground sensors and satellite data that provide more information about change across the landscape.

Using these datasets, we found that over the past 35 years, we have not seen significant changes in snow accumulation or total annual precipitation. However, when we looked at temperature data, we have seen significant May–September warming in the high elevation zones of the upper Henry’s Fork and Teton River subwatersheds where SNOTEL sites collect data. Late spring- and summertime warming increases atmospheric evaporative demand (also known as atmospheric thirst), which dries out the landscape and can lead to an earlier start of the growing season. Higher evaporative demand and longer growing seasons affect how much snowmelt becomes streamflow versus evapotranspiration (ET; how much water evaporates or is used by plants).

Streamflow is the “extra” rain and snowmelt that does not get returned to the atmosphere through ET or go towards deep groundwater recharge. Therefore, understanding changes in streamflow requires understanding changes in ET.

Changes in vegetation can also affect how much snowmelt goes towards ET. Using land cover data, we found that trees covered more of each subwatershed in 2020 than they did in 1989. This change in land cover is due to forest recovery following extensive disturbances including logging, fire, disease, and beetle infestations. The map below shows how tree cover has changed across the watershed with darker green colors showing areas with denser tree cover. As seen in the map, there has been the greatest forest change in the Upper Henry’s Fork.

tree cover graph

With large portions of the Henry’s Fork watershed experiencing warmer growing seasons and denser tree cover, it is no surprise that most of the forested regions of the watershed have seen significant increases in ET over the past 20 years.

Below you can see where there has been a significant trend in satellite-derived ET since 2002 (green). As with changes in tree cover, we see the upper Henry’s Fork has seen the strongest increasing trends. The following graph shows that there is a strong relationship between the average rate of ET in the Upper Henry’s and how much precipitation becomes streamflow. When ET is higher, less snowmelt and rain becomes streamflow.

Watershed Averaged graph
water trends graph

So back to the original question, why is the natural water supply declining in the Henry’s Fork?

Key Takeaways:

  1. Water supply declines across the Henry’s Fork watershed are largely driven by changes in the upper Henry’s Fork.

  2. Due to warmer growing seasons and denser forests, ET is increasing. This means more water is evaporating and forests are using more water in the soil and groundwater.

  3. This increase in ET has led to less snowmelt and rain becoming streamflow.

Have questions or want to learn more about what we do at Henry’s Fork Foundation? Contact us!

Facebook
LinkedIn
Pinterest
Categories
Recent Posts

Recent Posts

Person holding a rainbow trout.
Water Quality
Rob Van Kirk

Water Year 2025: Everything you wanted to know and a lot more!

Every year since 2017, the Henry’s Fork Foundation Science and Technology Team has produced an annual technical report that documents all of our data collection and analysis activities for the year. Water and irrigation year 2025 was a very active year for the team, given one of the hottest and driest summers on record, our 11th year of aquatic invertebrate sampling, and the 20th year of monitoring the rainbow trout spawning run in the Buffalo

Read More »
Rob Van Kirk April 1, 2026
Water Supply
Otto Lang

Monitoring the Upper Snake Snowpack: Updates from the Field

Hello! My name is Otto Lang, and I am a postdoctoral researcher with Boise State University and the Henry’s Fork Foundation. I’m working alongside Rob Van Kirk at the HFF, Lejo Flores, Professor in Geosciences at Boise State University, and Sarah Newcomb at Trout Unlimited. Together, we are in the midst of a two-year WaterSMART project funded by the U.S. Bureau of Reclamation to improve predictions of snow water supply across the Upper Snake River

Read More »
Otto Lang March 27, 2026
2 people in gear taking water samples
Water Quality
Rob Van Kirk

Henry’s Fork Aquatic Invertebrate Trends, 2015-2025

To address angler concerns about decreased hatches, particularly at Last Chance and the upper Ranch, and holistically assess ecological function throughout the mainstem Henry’s Fork, we implemented a statistically rigorous, quantitative study of aquatic invertebrates in 2015. This followed implementation of a watershed-wide water-quality monitoring program in 2014 and prompted subsequent expansion of stream gaging. After our 11th year of sampling in 2025, we had a set of 56 independent observations of macroinvertebrates collected at

Read More »
Rob Van Kirk March 23, 2026
Water Quality
Rob Van Kirk

Fish of the Month: Year 11

As it is the afternoon of December 31, I am closing the year with my annual fish-of-the-month blog, this one at the end of my 11th year of catching a fish in my home waters in each month of the year. To spare you the math, that’s 132 consecutive months of catching a wild trout or whitefish (and in some years I’ve fair-caught suckers and shiners, too) within a three-hour drive of my home in

Read More »
Rob Van Kirk December 31, 2025