A Renton area resident has written to the council a few times asking us to please dredge the mouth of the Cedar river. The resident has pointed out that sediment has built up a new shallow delta, and logs from recent river flooding have covered the area. The end result is a hazard to boating, as well as an apparent increase in lower-river-stretch flood risk.

But, we’ve had to inform him that we are unlikely to dredge this section of river anytime soon. We tried this in the early 1990’s, and it turned out to be highly expensive and largely ineffective for long-term flood relief. If I recall correctly, city taxpayers shelled out more than half a million dollars, and the state more than matched this amount, for an effect that seemed to disappear after the next flood season refilled the lower river stretch with new sediments.

It’s physics… when a flowing river runs into a still body of water, it drops its entrained sediment.

We did finally give ourselves flood relief in this zone. We built flood walls and berms the reach several feet above the high water level, and several feet above the elevation of the lake, so that we can keep the cedar river contained and flowing fast all the way until it reaches Lake Washington– regardless of how much sediment it deposits along this stretch of river bottom.

With regard to the logs, we’re working with other governmental agencies to get rid of those. The Mercer Island Police, who provides marine patrol services to Renton under contract, will occasionally tow dangerous floating logs, one at a time, to a temporary holding facility. But it’s up to the state to ultimately keep the waterways safe.

Here are some pictures I shot of the current situation:

The Cedar River dumps into Lake Washington right below the Boeing airplane bridge, seen above.

Logs and sandbars populate the area around the Cedar River delta, creating a hazard for unwary boaters

These logs get washed down by the Cedar River from upstream in Maple Valley or Renton, and left in the mud and sand at the river mouth. Department of Fisheries likes the habitat provided, but boaters can get in trouble if they are not careful.

And here is an email our Public Works Director sent to the City of Seattle last week, with additional information on this topic:

: Gregg A. Zimmerman
Sent: Wednesday, April 01, 2009 7:52 PM
To: Pat.Lee@Seattle.gov
Cc: fmandrews@aol.com; Don Persson; Jay B Covington; Lys L. Hornsby; Ronald Straka; Julia Medzegian
Subject: Cedar River Access Facility

Dear Pat Lee:

I have been following the e-mail exchanges between FM Andrews and you regarding the question of dredging the Cedar River delta. I just wanted to add a couple of clarifying points from the City of Renton’s perspective.

Over the last 15 years the City of Renton has been involved in two large dredging projects, one within the Cedar River Delta and the other along the lower channel of the Cedar River.

The earlier of the two projects was the dredging of the Cedar River delta. The delta dredging project had an unsatisfactory outcome in several respects. Renton was not able to obtain the required permits for important elements of the project due to concerns raised by the environmental resource agencies and the Muckleshoot Indian Tribe. The City of Renton went forward with the delta dredging portion of the project even though these other key elements had to be abandoned. Dredging of the Cedar River delta was extremely expensive and did not prevent or alleviate flooding of the river in Renton during high river flow events. In addition, within several years sediment transport in the river caused the delta to re-form. The City determined that the costs of this project far exceeded the benefits, and decided that it was not in the City’s interests to dredge the delta again.

A few years later Renton partnered with the Army Corps of Engineers in the lower Cedar River Flood Control project that included dredging of the lower channel of the Cedar River and the construction of flood walls and berms along the banks. This second project has proven to be a highly effective flood control project, as evidenced by the fact that the river did not overflow its banks during the severe January 2009 flood event. The City remains committed to periodic maintenance dredging of the lower channel of the Cedar River in order to preserve channel capacity and to prevent future flooding.

This maintenance dredging does not include the Cedar River delta. With the exception of a project (currently unfunded) to dredge a small portion of the lake at the north end of the Renton Municipal Airport in order to improve safety for sea plane operations, the City of Renton has no plans or funding for future dredging of the Cedar River delta. Furthermore, the City of Renton does not have jurisdictional authority or responsibility for dredging the Cedar River delta.

I hope these clarifications have been helpful.

Sincerely,

Gregg Zimmerman, P.E.
Public Works Administrator
City of Renton
1055 South Grady Way
Renton, WA 98057