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

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?

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 that 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.

Henry's Lake mountains, photo credit Sarah Newcomb

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.

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.

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.