Streamflow Forecasts above Average for Spring and Summer
After low flows in some of Montana’s rivers last summer caused issues for irrigators, anglers and recreationists, the spring and summer runoff this year looks to yield above average streamflows, according to snowpack data released by the USDA Natural Resources Conservation Service (NRCS).
Snowpack across the state is above normal for May 1 in all but a few sub-basins. Basins west of the Divide, which typically peak during the month of April, are all above normal with high elevations still gaining as of the end of the month. East of the Divide, where snowpack at higher elevations typically peaks a bit later towards the end of April to mid-May, also saw excellent gains during the month.
“Last month there was some concern over the lack of snowpack in some basins east of the Divide that provide irrigation and municipal drinking water, but April provided relief via abundant mountain snowfall and valley precipitation,” said Lucas Zukiewicz, NRCS water supply specialist for NRCS in Montana. “Two of these basins in southwest Montana, the Ruby River Basin and Hyalite drainages, have snowpack that is now normal for April 1.”
Abundant precipitation throughout this water year in most of Montana and healthy snowpack totals on May 1 have resulted in streamflow forecasts that are above average for most rivers in the state. In addition, the melt at higher elevations has been slightly delayed in some basins this year due to the cool and wet weather experienced during April.
“Delayed onset of snowmelt generally provides more efficient runoff and helps to keep the water in the mountains until it is needed to sustain streamflows later in the summer,” Zukiewicz said. “Over the last three years there has been early runoff of the seasonal snowpack, which has led to below average flows later in the season.” Streamflow forecasts issued by the NRCS are duration forecasts, or the total amount of water that will pass by a streamflow gauge during runoff season and do not forecast timing or magnitude of flows on any given day.
“The words ‘too much snow’ don’t come out of my mouth very often, but with regards to the snowpack in Wyoming basins, which feed the Bighorn River, it’s the case this year,” he said. Snowfall in the Wind River and Shoshone River basins has been record breaking this year, with snowpack totals over 200 percent of normal in some areas on May 1. Federal water managers have been working to make room for the water that will enter the river systems and reservoirs during runoff this year, increasing outflows from reservoirs in Montana and Wyoming. The May 1 – July 31 seasonal volume forecasts for some of the rivers in Wyoming are approaching record levels, with some over 200 percent of average. Zukiewicz said water users should anticipate above average flows for some time on the Bighorn River.
Monthly Water Supply Outlook Reports can be found here after the 5th business day of the month:http://www.nrcs.usda.gov/wps/portal/nrcs/main/mt/snow/waterproducts/basin/
May 1, 2017 Snow Water Equivalent
|River Basin||Percent of Normal||Percent of Last Year|
|Columbia River Basin||119||163|
|– Kootenai in Montana||121||178|
|– Flathead in Montana||125||169|
|– Upper Clark Fork||109||140|
|– Lower Clark Fork||124||185|
|Missouri River Basin||108||144|
|– Headwaters Mainstem||90||130|
|– St. Mary-Milk||124||200|
|Yellowstone River Basin||157||174|
|– Upper Yellowstone||137||180|
|– Lower Yellowstone||172||174|
|West of the Divide||119||163|
|East of the Divide||133||166|
May 1, 2017 Precipitation
|River Basin||Monthly Percent of Average||Water Year Percent of Average||Water Year Percent of Last Year|
|Columbia River Basin||124||130||129|
|– Kootenai in Montana||142||141||128|
|– Flathead in Montana||146||136||135|
|– Upper Clark Fork||105||113||119|
|– Lower Clark Fork||123||136||135|
|Missouri River Basin||126||128||125|
|– St. Mary-Milk||105||149||123|
|Yellowstone River Basin||168||153||150|
|– Upper Yellowstone||140||145||154|
|– Lower Yellowstone||181||161||152|
|West of the Divide||124||130||129|
|East of the Divide||137||137||138|
May-July 50% Exceedance Forecasts
|River Basin||Highest Point Forecast*||Lowest Point Forecast**||Basin Avg Forecast***|
|Upper Clark Fork||136%||102%||125%|
|Lower Clark Fork||130%||119%||125%|
|Yellowstone River Basin||244%||94%||153%|
Note: Streamflow forecasts are issued for multiple points on rivers and streams within a major river basin and are given as a range of exceedance probabilities. Consult the individual river basin of interest to see the range of values for streams of interest.
*Highest point forecast is the highest 50% forecast of all forecast points within the basin.
**Lowest point forecast is the lowest 50% forecast of all forecast points within the basin.
***Basin average forecast is an average of all 50% forecasts within the basin.