By Marcy Stamper
Heavy rains appear to have contributed to the failure of three Wenner Lake dams, adding to the volume of water that turned the normally modest Benson Creek into a torrent Thursday night (Aug. 21).
Wenner Lakes are a series of five ponds used for irrigation and recreation tucked among the hills between Benson Creek and Finley Canyon. Normally contained by earthen dams, three of the ponds gave way, according to the Washington Department of Ecology, which inspects and regulates the dams, along with 1,000 similar dams throughout the state.
While a definitive explanation for the breached dams is not yet available, Ecology believes that a combination of factors, including a high volume of stormwater runoff, caused the breaches, according to Dan Partridge, communications manager for Ecology.
Thursday night’s storm dumped 2 inches of rain on severely burned areas that no longer have vegetation to catch the stormwater. Dead trees and debris may also have clogged the dams’ spillways, elevating water levels beyond the dams’ capacity, said Partridge.
The dams—three in private ownership and two owned by the Washington Department of Fish and Wildlife (WDFW)—are fed by a Finley Canyon tributary of Benson Creek.
One of the private dams, called the Hawkins Dam, collapsed during the storm, although part of it is still standing and about 40 percent of the water remains in the pond, according to Partridge. At four acres in size, Hawkins is one of the larger dams and, had all the water been released, it would have been equivalent to a football field covered by 40 feet of water.
The other two dams that collapsed are the smallest of the five, one in private ownership and one owned by WDFW.
Ecology had two engineers at the site all weekend as contractors cleared debris from spillways and repaired damage from the flood. They also shored up the spillways so that they will be better equipped to handle a large volume of water, said Partridge. The work is being paid for by the dam owners.
The three private dams are all protected by conservation easements for wildlife habitat held by the Methow Conservancy, which has been monitoring the situation, according to Executive Director Jason Paulsen. Although Wenner Lakes are contained by constructed earthen dams, they essentially look like natural ponds, he said.
The dams are on a five-year inspection cycle and were due for inspection next year, said Partridge. The two smallest dams are not regulated by Ecology since they hold less than 10 acre-feet of water.