Wall of Willows
Posted By Dwight on July 22, 2010
It appears to me that work which was done at the Sammamish weir in 1998 may have had the unintended consequence of plugging the outlet to Lake Sammamish.
It is clear to me that the vegetation below the weir is now the critical factor for draining the lake, this vegetation is more restrictive than the weir itself in all but the lowest summer flow rates.
How could this have happened? Here is my take on it:
Prior to 1998 the weir was at elevation 25.5, mostly level with a wide and shallow outlet to the transition zone. The low flow channel just downstream of the weir was open and about 50′ wide (my estimate from the 1991 photo posted below).
Two improvements for fish passage in 1998 narrowed the low flow channel; a notch was created in the center of the weir, and a log sill with a 4′ wide notch was added just below a small fish resting pool below the weir. It seems that these two improvements narrowed the low flow channel and allowed the willows to grow together into the channel in the low flow summer months. Now it is hard to identify any low flow channel at all. The view from the weir downstream is a “wall of willows” where the low flow channel once was. This has become the limiting factor for the outflow of Lake Sammamish in low, high, and normal flow conditions. It is great to do things which are good for fish passage, but this work may have also caused lake levels to be substatially higher. That was not part of the plan.
I hope that this situation will be quickly rectified so that the lake flood levels can be better contoled next year.
Comments