Volcanic debris continues to wash down the Toutle and Cowlitz Rivers from Mount Saint Helens. In spring and winter, high turbidity caused by suspended sediment significantly reduced plant capacity. Turbidity in the river can exceed 2,000 Nephelometric Turbidity Units (NTU) during spring run-off and winter storms. Turbid raw water significantly slows the treatment process in order to produce finished water less than 5 NTU to meet drinking water standards.
In the summer, rising sand bars and low water levels threatened to leave the intake dry. In 1986, the U.S. Army Corps of Engineers (USACE) constructed a dam on the Toutle River to capture sediment before it reached the Cowlitz.
By 1998, the dam was full and water began coming over the spillway, bringing silt, sand and sediment with it. In 2002, the USACE projected the Cowlitz River bottom would raise 9-feet at the City’s intake structure by 2034. But four years later, the river had already filled in roughly 12-feet at the mouth of the Cowlitz.
In 2005, the City constructed its own 8-foot sediment dam in front of the intake to keep it from being silted in but the dam was overtopped the next year. The USACE dredged the lower Cowlitz but funds are not available to dredge far enough upriver to reach the intake structure.
The City’s dredging permit to maintain a sump in front of the intake structure and keep it clear of sediment has expired; renewal will require the intake to be upgraded to meet current fish code. The size of the openings in the intake screens is too large to keep out smelt larvae and recently hatched fish. The structure also lacks a fish return to send entrained fish back out to the river, and the flow velocity at the face of the intake screening is too high. Screen failures due to sediment build-up against the screens were already problematic; smaller screen openings would create more plugging problems because coarser sediment would not be able to pass through.