Combined releases to increase but flood risk remains normal
May 10—JAMESTOWN — Combined releases from Jamestown and Pipestem reservoirs will increase to 1,300 cubic feet per second by Wednesday afternoon, May 10, after some recent precipitation but the risk for flooding remains normal or potentially slightly elevated in the Jamestown area, according to Allen Schlag, hydrologist with the National Weather Service in Bismarck.
"Water will be abnormally high for a while as the dam owners work to get their reservoirs back down to what we call full supply level," Schlag said.
He said the actual risk is low for problematic high water that threatens roads or significantly impacts agricultural interests.
"We have seen the water remain abnormally high for the foreseeable future, but as far as the risk of flooding, I would say that's probably pretty close to normal, maybe even slightly elevated," he said.
A 3-inch rainfall north of Jamestown and Pipestem reservoirs will result in an increase in combined releases from 1,000 cfs to 1,300 cfs by Wednesday afternoon, said Alex Flanigan, water manager of Pipestem Dam for the U.S. Army Corps of Engineers, in an email. Combined releases increased from 1,000 cfs to 1,100 cfs on Monday, May 8, and were expected to increase again to 1,300 cfs on Wednesday afternoon.
Releases from Jamestown Reservoir increased from 700 cfs to 800 cfs on Tuesday, May 9, and were expected to increase again to 900 cfs on Wednesday afternoon.
Releases from Pipestem Reservoir increased from 300 cfs to 400 cfs on Monday.
Flanigan said the most probable peak combined release range from the two reservoirs is 1,300 cfs to 1,600 cfs.
"We will monitor reservoir inflows this week along with the rain forecasts," he said.
The water levels at Pipestem Reservoir rose by more than 2 feet since Friday, May 5, to 1,467.5 feet above mean sea level, said Jeremy Thury, Pipestem Dam manager for the Corps. He said the inflows into Pipestem Reservoir increased to 950 cfs, an increase of 300 cfs since Friday, mostly from the precipitation over the weekend.
The water levels at Jamestown Reservoir increased from 1,440.3 feet AMSL to 1,441.1 feet AMSL since Friday, he said.
With the recent and forecasted precipitation in the Jamestown area, Schlag said he anticipates seeing minor rises from the rivers upstream from Jamestown and Pipestem reservoirs but nothing significant.
"For example, the James River near Grace City, it's going to rise back up into what we call action stage," he said. "Not a real damaging high water event by any stretch of the imagination, but what it will do of course, it will put more water into the Jamestown Dam, similarly we expect to see a little more inflows into Pipestem as well."
Schlag said the area is transitioning from springtime rain or stratiform rain — generally non thunderstorms and a gentle, long duration widespread event — to thunderstorm season.
"So now we are starting to see a little more thunder and lightning involved and typically speaking that increases the rainfall rate," he said. "When we put an awful lot of water into the ground this spring through snowmelt and through stratiform rain because when the rates are very low in the rainfall, most of it ends up in the ground, but as we go forward when thunderstorms hit an area, they dump a lot of rain in a short amount of time and that does increase the amount of runoff regardless of the soil type."
He said he isn't concerned about too much risk of damaging high water unless there is a large thunderstorm that drops 3 to 5 inches over a large area.
"I think if you saw one 3 to 5 inch rain like I say over a really broad area and these rainfall rates you know you get your 3 to 5 inches in a couple of hours, again, that really enhances runoff when those rainfall rates are high," he said. "That is where you see the big runoff events."
