Snowpacks and Runoff

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Bighorn Pass, upper Gallatin River watershed – Mark Story

(June 7, 2018) Written By Mark Story, GGWC Board Member – The year 2018 has had very robust snowpacks throughout much of Montana with end of May snowpack water equivalents ranging from 87% of average in NE Montana to 206% of average in NW Montana.  As of June 7th, the Gallatin River drainage is currently at 120% of average.  Approximately 60% of the streamflow in the Gallatin Watershed occurs from snowmelt with snowmelt peak flows occurring in May and June.  As a “rule of thumb”, snowmelt peak flows occur when about 50% of the mountain snowpacks have melted.  Throughout much of the Gallatin watershed, snowmelt does not directly run into stream channels, but recharges ground water tables which then connect and drain to surface streams.  The Gallatin watershed could be thought of as a sponge which gets soaked during snowmelt, then gradually wrung out during the summer and fall with some groundwater recharge from summer and fall storms.   Winter streamflow in the Gallatin drainage is almost entirely attributable to groundwater drainage.

Snowmelt and subsequent runoff provide many useful ecological functions such as charging reservoirs and both shallow and deep groundwater aquifers.  The conveyance systems of groundwater flow and surface streams are very complex.  During peak snowmelt, much attention is focused on large stream systems and flooding.

The photo above was taken on the Clarks Fork River near Missoula which was running at 42,000 cfs on May 20, 2018.  Note that the river is well out of the stream channel and flowing through much of the floodplain.  Floodplain inundation provides many beneficial functions such as peak discharge moderation, groundwater recharge, nutrient loading to floodplain vegetation, enriching wildlife habitat, stimulating “disturbance” of floodplain vegetation which promotes regeneration, and many other valuable ecological benefits.

When development occurs in or near floodplains, such as in the subdivision above on the Clarks Fork River, the floodplain functions are considered “damaging”, frequently resulting in floodplain modification attempts like rip rap installation or flow diversion.   At present, urban “encroachment” on floodplains in the Gallatin watershed is relatively limited, with the major exception of the eastern part of downtown Bozeman and subdivisions along the creek south of Bozeman, which are located within the Bozeman Creek floodplain.

Numerous studies of snowpacks and streamflow regimens in the Western US have consistently documented a trend of decreasing mountain snowpacks and earlier snowmelt runoff which results in longer low streamflow periods, reduced irrigation water availability in late summer, and longer exposure of mountain forests to wildfire potential.  Long term climate projections indicate these trends are expected to continue so the snowmelt contribution to the Gallatin Watershed will likely decrease over time, with more late season streamflow and irrigation water shortages.  Several decades from now, Montana and Gallatin Valley residents may look back on years like 2018 as the “good old days” when snowpacks were sufficient to provide robust runoff events.