The reservoir at Howard Hanson Dam reached 1,147 feet above sea level the week of April 30, the point at which the U.S. Army Corps of Engineers began intensive monitoring, including scientists on the ground around the clock. The Seattle District of the Corps slowly began last month to allow water levels to rise behind the dam for the annual spring fill of the reservoir. Spring storage of water is a normal operation to provide summer and fall water supply for various purposes.

During the pool rise, the Corps conducted testing and in-depth investigations of the dam’s current effectiveness. The current plan targets a maximum pool elevation of 1,167 feet, unless engineers, geologists and other scientific experts on the ground see anything troubling, in which case the reservoir level can be reduced. Additionally, throughout the reservoir fill, the Corps monitored the performance of the dam including special electromagnetic and dye tests to measure the effects of the pool on the right abutment. Because of the damage sustained to the abutment in 2009, a higher frequency of monitoring is prudent once the pool exceeds elevation 1,147 feet.

Water stored during the spring refill of the reservoir will be used to provide water supply to the city of Tacoma and its water supply partners. Additionally, four-fifths of the reservoir’s supply will be used to augment downstream flows to help endangered species of fish, including Chinook and steelhead. Project managers believe the regulated downstream flows will aid spawning and other life cycle events for the fish.

More details and history about Howard Hanson Dam and the right abutment issue can be found at www.nws.usace.army.mil.

http://www.nws.usace.army.mil.