Beavers have been here a many years. King County has published technical papers on managing beavers as well has a handy summary chart and many other resources. Several initiatives have been made over the years to mitigate the flooding problem. I have collected as much information as I can on this from long term residents who have been working on this problem for several decades. Please let me know if I need to make any corrections.
University of Washington Ecology Study
May 2006 an ecology study published by UW ecology students studied the tree and forest of the fen (wooded area owned by King county that borders the lake). This study suggested that the health of the trees and forest was in decline due to flooding from increasing water levels.
King County also acknowledged damage to the fen, likely from flooding but at the time, they took a laissez-faire (meh) attitude about this.
Help solicited from King County
Financial help in solving this problem was requested from King County in 2006 which was declined. King county has 3 factors which are required to spend funds. While they did study the matter and found 2 factors in play, the 3rd factor, damage of a dwelling or creation of a public health hazard was not satisfied by King county so no financial help. That said, they did work with on permission and access.
Beaver Trapping
We hired a trapper and trapped 20 beavers with King County Parks permission from 11/2005 to 4/2006. The beavers were relocated. It didn't take long for the beavers to return to beaver paradise. Typically beaver trapping is recommended in conjunction with beaver deceivers so they don't notice the change. This might have been part of a larger strategy.
Hydraulic Permit & Small Lake Levelers
We had a Hydraulic Project approval permit that was valid from 11/2004 to 11/2009. First year of work was for lowering the dams by hand a little at a time and occurred in 2005. The permit was modified in 2005 to include the use of lake levelers / beaver deceivers. 3 dam levelers were installed in 2005. A small team kept the deceivers clean and working for several years afterwards. The pipes used were landscape drainage pipe and were easily plugged with mud by the beavers. These levelers were effective at controlling the lake level for quite a few years as seen on the long term levels chart. Maintenance eventually stopped and the levelers stop functioning.From community notes in May 2005:
A beaver deceiver was installed May 9th per Hydraulic Project Approval. "This consisted of one [4"] pipe with a cage around the inlet in dam 3 (largest dam and furthest downstream) and just a pipe in dam 2. The deceivers were installed in this way to minimize the cost with the hope that it would work. On May 14th a work party added a second pipe to dame 3 (2 pipes now with a cage at their inlet) and added a pipe to dam 1. A work party will modify the deceiver on [June 10th] to add cages to the pipe inlets at dam 1 & 2."
We were also granted permission to notch the dams but it has restrictions including we can’t release water between 9/15 and 1/31 due to it being spawning season and there are some other rules about heavy rains etc. in the permits.