35th Anniversary — Lessons Learned.


Part 4: Hydrology


Photo by James Chandler.


Deep aquifers on the Yellowstone Plateau are the primary source of streamflow in the upper Henry’s Fork subwatershed (i.e., the watershed upstream of Ashton). Big Springs, Warm River, and Buffalo River are the three largest spring-fed tributaries to the Henry’s Fork. Snow falls on the Yellowstone Plateau, then recharges the aquifer as it melts. Due to the thickness (up to 1,000 feet) and geology of the aquifer, it takes 50-100 years for an individual water molecule to make its way through the aquifer and emerge at the springs. Fortunately, the response time of flow from the springs is much shorter, around 3 years. This response is due to the pressure of snowmelt entering the aquifer at the top and pushing water that’s already in the aquifer out the bottom at the springs.


Here are three key lessons learned about the hydrology of the Henry’s Fork:



1.  Response time of flow at Big Springs to precipitation on the Yellowstone Plateau is about 3 years. Water flows out the bottom of the aquifer at Big Springs depending on pressure at the top of the aquifer from most recent year’s snowmelt.


2.  That is why streamflow in the upper Henry’s Fork this year depends on the snowpack over the last three years.


3.  For example, after four years of drought from 2013-2016 we had a good water year (2017), but flows out of Big Springs did not “bounce back” that same year. In 2019, after three good water years, we will see flows from Big Springs return to average.


Photo by Jeremy Giampaoli.


Learn more about streamflow in 2017 here.