35th Anniversary – Lessons Learned.


Part 5: Island Park Reservoir


Many already have a sense that Island Park Reservoir plays a significant role in water quality, streamflows, and the fishing experience downstream, but here are 4 key lessons learned about the reservoir since HFF’s founding in 1984:



1.  HFF’s research on effects of Island Park Reservoir on water quality and the fishery downstream shows that keeping the reservoir as full as possible for as long as possible results in cooler summer-time water temperatures below the dam, lower turbidity, lower suspended sediment export from the reservoir, and higher winter flow. Thus, mid-summer fishing experience and long-term habitat quality and trout recruitment all benefit from a full reservoir.


Low storage levels in IP Reservoir lead to higher turbidity downstream of the dam.


2.  A full reservoir also increases the chance of a beneficial spring freshet that can move sediment out of certain river reaches and provides habitat benefits for trout and aquatic insects all the way down to St. Anthony.



3.  A higher summer reservoir level provides more summer habitat for fish in the reservoir, which contributes more fish to the upper river.


4.  When the reservoir has more water, there is more cold water that sinks to the bottom — called thermal stratification. When the reservoir stays full, it stays stratified longer and has more depth in the water column; as light can only penetrate so far, algae and cyanobacteria growth is limited to the upper layer that receives sunlight.



Learn more about Island Park Reservoir and cyanobacteria blooms here.