Snake Basin reservoir

The American Falls Dam. Near-record snowfall in February, particularly at higher elevations, left many river basins in Southern Idaho and Eastern Oregon with above-average snowpack.

Reservoir levels and soil-moisture profiles look plentiful in much of central and southern Idaho as crop irrigation and livestock grazing seasons ramp up, speakers said at an Idaho Water Supply Committee meeting April 11 at state Department of Water Resources headquarters in Boise.

Near-record snowfall in February, particularly at higher elevations, left many river basins in Southern Idaho and Eastern Oregon with above-average snowpack. Low- and middle-elevation snowmelt flooded some areas as April began. Melt is yet to occur at many high-altitude spots, which continue to get new snow.

An estimated 119% of average runoff is expected in the Boise River Basin, said hydrologist Ryan Hedrick of the U.S. Bureau of Reclamation Middle Snake Field Office. Lucky Peak, Arrowrock and Anderson Ranch reservoirs are 66% full as flood-control releases continue. The reservoirs have ample time and remaining snowpack to fill for this year, for irrigation and other uses, he said.

Owyhee Reservoir near Adrian, Ore., is 71% full. Though it won’t fill unless the spring is very wet, it should supply ample irrigation water this year, he said. It was 56% full in mid-July. Snowpack in the basin stands at 125% of normal, compared to just over 30% a year ago.

Snowpack is above normal in most basins in Idaho’s central and southern regions, and slightly below long-term averages in the north.

Above-average snowpack is good for irrigators, and so is the recent heavy rain in much of Southern Idaho, said Ron Abramovich, USDA Natural Resources Conservation Service water supply specialist in Boise. Rainfall, to the extent seen recently, saturates the soil profile for the start of crop-growing and rangeland grazing seasons, enabling producers to save water.

This year contrasts with the dry spring of 2015, when irrigation demand was about three times the norm in May of that year, he said.

Recently, “the rain we saw provided deep penetration of water into the soil profile,” Abramovich said. “That is much better than the way a center pivot (sprinkler) can fill the soil profile.”

Troy Lindquist, National Weather Service senior hydrologist in Boise, said forecasts for May through July favor above-normal precipitation.

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