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FAQ: What is a Spring Freshet?

Updated: May 9, 2023



NEW TOOL: Track turbidity during the 2023 Spring Freshet!


Freshet:


A high rate of streamflow for a short amount of time. Typically a natural process in rivers caused by heavy rain or rapid snowmelt.



The Two Freshet Types on the Henry's Fork


On the Henry's Fork, a freshet can be:

  1. Natural: from heavy rain or rapid snowmelt into a full or nearly full Island Park Reservoir, requiring a rapid increase in outflow from the dam

  2. Managed: a planned freshet, negotiated with water managers and decision makers, to achieve (or even improve on) the benefits a natural freshet brings


What are the benefits of a spring freshet?

  • Transports sediment out of the river between Island Park Dam and Pinehaven (most notably, the Ranch)

  • Improves habitat for key aquatic insects and thereby has the potential to improved hatches

  • Improves 2 critical measures of insect habitat quality: 1) %EPT (mayflies, caddisflies, stoneflies) and 2) HBI (Hilsenhoff Biotic Index)


What have HFF and partners achieved so far through spring freshets?




  • Improvement in percent EPT (mayflies, stoneflies, and caddisflies), and HBI (measure of aquatic habitat quality), at Last Chance and Osborne, comparing 1993 (post Island Park Reservoir sediment event) with 2015-2022.


  • % EPT (mayflies, stoneflies, caddisflies) at Last Chance and Osborne measurably increases one year after a high springtime freshet flow (i.e., the best observed HBI score was in 2019, after the 2018 freshet).


What conditions improve habitat the most?

  • The larger and more rapid the increase in streamflow, the larger the amount of scour and sediment removal

  • Pairing a freshet with relatively low irrigation-season flows, as lower irrigation-season flows leads to lower deposition of sediment

  • Ensuring the high freshet flow occurs prior to significant aquatic plant (macrophyte) growth that would trap and prevent removal of sediment from the stream bottom (earlier in the spring)


Why can't we get a spring freshet every year?

  • Spring freshets are not part of water or reservoir managers management priorities. Their mandate/requirement is to manage the reservoir for irrigation need (and save water until needed for irrigation), and to avoid flooding.

  • A spring freshet is only possible if there is enough water available (i.e., without saving over 40,000 acre-feet of water in Island Park Reservoir in 2022, there would not be water available for a possible freshet in 2023, even with above-average snowpack).


Will a spring freshet harm Rainbow Trout spawning on the Henry's Fork?


Thankfully, no.

  • Trout spawning success revolves around there being enough aeration within the gravel for eggs to survive and hatch, without there being so much flow that the redd is buried due to channel mobility.

  • The goal of the freshet is not to move gravel, but to move sediment finer than a grain of sand.

  • Widespread destruction of redds would require complete rearrangement of the channel, as in entire gravel bars move dozens of meters, the river cuts a new channel, etc.

  • The last time outflow reached 2000 cfs at IP Dam was in 2011, when it was at or above 1900 cfs for 13 days in late May and at or above 2000 cfs for 7 of those days. (A lot more eggs and fry would be in the gravel in late May than will be there in late April of this year.) Two years later, the fish from that spawn made up the largest cohort of two-year old recruits into the population on record.

  • Freshets on the South Fork have not been effective in reducing Rainbow Trout populations, which is why other methods are being explored by IDFG.

  • By way of monitoring long-term effects, HFF will compare ADCD-measured stream channel cross sections this year with those collected last year--at the same time of year to control for macrophyte growth--to see if the channel has changed at all between the two years. We can also estimate channel changes via the USGS gage shift.

What size sediment does the freshet move?

  • Fine material sitting on the stream bottom, not the streambed itself where eggs are currently in the gravel.

  • Research over the past few decades shows that the Henry's Fork lacks the stream power to mobilize gravel and cobble on the streambed (even at flows of 3,000+ cfs)


If there is enough water for a spring freshet, why couldn't that water have been used for winter flows?

  • Winter flow management is set in November, when so much can still change (as we've seen in past years), and weather is notoriously hard to predict (just ask your local weatherman!)

  • Not until March was there enough snowpack (water) to make a freshet possible (enough time to dig out of the record low baseflows hole)

  • On the bright side, conservation work with partners last year allowed for a boost to winter flows of 100+ cfs over and above natural conditions (adding ~550 trout to the population).




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