WENATCHEE, WA.- Icicle and Peshastin Irrigation Districts at their March 13th board meeting declared an emergency as pertaining to the dam at Eight Mile Lake. The emergency condition is not due to a change in the condition of the dam but due to the Jack Creek Fire from last fall. The Jack Creek fire burned a significant portion of the watershed above the lake. Some of this area burned very hot damaging the soils and reducing their absorbency. The concern is that due to this fire water runoff from snow melting and/or rain events will be much higher than normal.
The threat of increased runoff above the lake was brought to light by a report released by the forest service that investigated the severity of the conditions above the lake. This report was then provided to the Dam Safety Office within the Department of Ecology and they ran the numbers and notified the districts that they would be raising the hazard level of the lake. Given the size of the lake and water shed above the lake if the dam suffered a catastrophic failure we could see flows as high as 20,000 cubic feet per (CFS) second or 8,976,000 gallons per minute come out of the lake. The historic high flows in Icicle Creek are around 15,000 CFS. If the dam were to fail, it would most likely fail during a high-water event such as one that would put the normal flow within the creek at over 10,000 CFS; adding the 20,000 CFS from a catastrophic dam failure and you could double the highest recorded flows.
The dam was originally constructed in the late 1920’s and is in poor condition. The districts have been contemplating the repair/replacement of the dam for some time but due to many factors have not made major repairs to the dam. This lake is within the wilderness and repairs present numerus challenges. The dam at the lake when originally designed was designed with an inadequate spillway; it being too small. The dam is constructed with concrete, mortar/rock, and a large earthen section. The earthen section of the dam has been over topped and part of it has washed away over the years creating a lower spillway that has worked to release some of the pressure on the main hardened portion of the dam. This area has been very stable, so the district has been postponing the repair.
Dam in jeopardy of catastrophic failure
With the recent fire and the possibility of much higher than normal runoff from the watershed above the lake the district feels that the dam is in jeopardy of a catastrophic failure. The districts do not feel that a dam failure is imminent or even likely but very possible. Due to the increased threat of a dam failure the districts have decided that they can no longer wait but must replace the dam and its outlet works. The districts are working at an accelerated rate to try to get the dam replaced this year.
The districts have already, this year, been up to the lake and have done what they can to reduce the threat of danger. The lake is currently about 7 feet below the full mark. The districts plan on lowering the lake another 13-15 feet before the major runoff happens. The districts are creating an Emergency Action Plan with assistance from Chelan County Emergency Management, Ecology’s Dam Safety Office and the US Forest Service and will notify potentially affected property owners.
Repair work Challenges
The districts have been reluctant to make any major repairs to the lake because the lake is within the wilderness. The existence and use of this reservoir is of the highest importance to the districts and our watershed. It makes it possible for us to grow the crops that we do within the Wenatchee Watershed and improves instream flows. There are currently seven reservoirs within the wilderness, all of them were created before the wilderness was created. They represent the only significant water storage within the Wenatchee Watershed. Of these reservoirs Eight Mile Lake has the highest refill ratio; it is the most likely to refill even during an extreme drought.
The districts have put the repair/replacement of this dam off for as long as it could but must now act. The districts will file a project level SEPA to the Department of Ecology within the next week. Comments or questions can be addressed to the Icicle Irrigation District, P. O. Box 371, Cashmere WA 98815.