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Both bridges are undersized in relation to the size of the channel. As a results they cause more flooding to occur on the upstream side of the bridges, with less flooding occurring downstream of the bridges. Both the 276th and 268th Ave SE bridges are undersized in relation to the size of the channel with concrete abutments that extend into the channel. When Boise Creek is full of water the bridges cause the water on the upstream side to slow down and stack up into a backwater that spills over the tops of the banks. On the downstream side, the bridges create a ‘shadow’ zone where the water surface elevation is lower, or shadowed, and where flooding is less frequent.
If the 276th and 268th Ave SE bridges were removed altogether the depth and frequency of flooding downstream of the bridges would increase.
Wider bridges at 276th and 268th Ave SE would reduce flooding along Boise Creek if the channel between and downstream of the bridges was also widened.
The King County Flood Control District prioritizes capital projects on the basis of flood risk. Flood risk refers to the consequences, severity, extent, and likelihood of the flooding problem. The Flood Control District has not yet received any Boise Creek flood risk reduction project proposals. However, two buyouts (one funded by Conservation Futures, River Improvement Fund, and 1989 Open Space Bond; the other funded by FEMA for repetitive loss) have occurred along Boise Creek in the past. Proposals are typically submitted by cities and King County through Basin Technical Committees. In the spring of each year the District’s Board of Supervisors determines whether new project proposals will be solicited for consideration in the next year’s budget. The District also has established a Flood Risk Reduction fund of approximately $2.7 million per year to help address local flooding problems.  


Dredging of streams regulated by federal, state and county laws and requires permits. Dredging is generally considered a last resort to manage flooding because it is short-lived and can destroy salmon nesting areas, reduce sheltering areas for juveniles, and can increase downstream erosion.
Dredging is possible, but permits are required. Contact the King County Agricultural Drainage Assistance Program (ADAP) for assistance.  Guidance can be found in the Manual of Best Management Practices for Maintenance of Agricultural Waterways in King County (Acrobat pdf). King County partnered with farmers, regulators, tribes, fish interests, the King Conservation District and others to develop this manual which provides a comprehensive and effective approach to ditch dredging. The goal is to protect water quality and fish habitat while streamlining regulatory requirements, reducing county costs, and adequately draining fields for farming. For more information, refer to King County’s best practices manual for waterway maintenance.
King County Department of Transportation, Road Maintenance Section
155 Monroe Ave. N.E., Renton, WA 98056
800-KCROADS (1-800-527-6237); Division 4 Lead: Gary Bachmeier

Ownership of Boise Creek

The water in Boise Creek belongs to the public, but the stream or river and banks belong to the people who own the adjacent land. Under the public trust doctrine, the water in rivers and lakes belongs to everyone and can be accessed through public rights of way.
Boise Creek is both a water of the U.S. and a water of the state. The U.S Army Corps of Engineers has federal jurisdiction over Boise Creek, and the Washington State Department of Fish and Wildlife has state jurisdiction over Boise Creek. Regulatory authority extends from high water on one bank to high water mark on the other bank.


Private property owners are mandated to control regulated noxious weeds on their property. Technical assistance is available from King County’s noxious weed control website.

Water quality

Turbidity in streams and estuaries is produced by particles, both organic and inorganic, suspended in the water column.
Turbidity has not been identified as a problem in Boise Creek. However, Boise Creek is formally identified by the state as having documented high-levels of fecal coliform bacteria.
Yes, damaging effects of turbidity are well-documented; it can affect migration behavior, spawning, growth, and reproduction of many fish species. Turbidity has been identified as a problem in the Upper White River (external site).


There are six species of salmon and trout in Boise Creek: Chinook salmon (King), Chum salmon (Dog), Pink salmon (Humpy), Coho salmon (Silver), Coastal cutthroat trout, Steelhead/rainbow trout. A bull trout has been observed but the status of this species in Boise Creek is not well understood.
Yes, Puget Sound Chinook (spring and fall types), steelhead, and bull trout are listed as ‘threatened’ under the Endangered Species Act.
Spring Chinook enter the White River in May and peak spawning occurs in August, which is a month or two earlier than fall Chinook. White River spring Chinook are the only remaining early Chinook run in South Puget Sound. While a significant number of Chinook spawners in Boise Creek are hatchery-origin fall Chinook, Boise Creek provides suitable habitat for both spring and fall Chinook.

Restoring stream habitat

Yes, the King County Small Habitat Restoration Program (SHRP) provides free technical advice and can work with you to design and implement habitat restoration projects on your property. For more information see King County’s Small Habitat Restoration Program website.
There are a variety of funding sources for improvements including, but not limited to: Washington State Salmon Recovery Funding Board, Puget Sound Acquisition/Restoration Fund, Floodplains by Design, King County Flood Control District, Cooperative Watershed Management Fund, King County Surface Water Management Funds. Boise Creek habitat restoration projects tend to score very highly due to the importance of White River spring chinook for Puget Sound Chinook viability. King County’s Boise Creek Habitat Restoration and Flood Attenuation Project is on a list of Floodplains by Design projects being proposed for funding by the state legislature in 2015-17.

Septic systems

Please refer to King County’s information for septic system owners.

Septic systems:  Septic systems or “on-site sewage systems” are comprised of septic tanks connected to septic drainfields.  The tanks allow the solid wastes to settle out from the wastewater before flowing to the drainfield.  Once in the drainfield, water filters through the soil.  This filtering action lets harmful bacteria be safely removed by natural bacterial processes.  Water that has filtered from a septic drainfield completely through the soil column ultimately rejoins the local waterway as a clean groundwater.  In this way, properly functioning septic systems help local natural waterways—creeks, streams, rivers and lakes-- be replenished with clean water, free of harmful bacteria and disease pathogens in human waste, as well as the fats, oils, greases and detergents associated with wash water.

Septic systems in Boise Creek: Septic systems are the only way residential sewage waste and wash water are treated in the Boise Creek portion of unincorporated King County.   In Boise Creek septic systems often don’t work in the ideal way described in the paragraph above.  Many Boise Creek septic systems are old (installed beginning in the early 1920’s) and many likely haven’t been well maintained.  Often these systems weren’t designed or installed professionally and with an approved permit from the Health Department.  Another major factor compromising healthy septic system functioning is that the groundwater is close to the surface, as the basin sits on hard compacted soil deposited during the Osceola Mudflow from Mt. Rainier thousands of years ago.  The high groundwater table means Boise’s soils are often completely saturated.  This prevents septic wastewater from being treated by the beneficial drainfield actions of filtering through the soil column and treatment by normal bacterial processes.

King County’s Involvement in Boise Creek Septic Systems: Under the current Municipal Phase I NPDES Stormwater Permit, which is administered by Washington State as required by the federal Clean Water Act, King County is required to seek failing septic systems (and any other land uses) that are causing high levels of bacteria to enter the County’s stormwater conveyance system that drains to Boise Creek.  For the last few years County staff have been investigating the area to comply with this Permit requirement.  Investigations have included performing desktop GIS analysis of air photos to look for any kinds of land use that could lead to high levels of bacteria entering the stormwater system.  Staff have driven all the roads in the basin and have walked much of the basin, observing the stormwater system and potential illicit discharges to it, including soil erosion, potential manure runoff and other concerns.  Staff have obtained water quality samples from 44 established locations in the stormwater system.  Directly related to this work, County staff found two septic systems so far in 2014 with wastewater and wash water flowing directly into the stormwater system, and from there directly into Boise Creek.  These illicit septic discharges are violations of the King County water quality code and Washington State water quality regulations.  Code and regulatory violations aside, most people would agree that having human wastewater flow into Boise Creek—including fecal waste, detergents and kitchen fats, oils and greases—is a bad idea and should be prevented by proper septic system installation and maintenance.

Noxious weeds

Private property owners are mandated to control regulated noxious weeds on their property. Technical assistance is available from King County’s noxious weed control website.

For more information about Boise Creek habitat restoration projects, please contact Mason Bowles, Senior Ecologist, WLR Ecological Restoration and Engineering Services Unit.