Lake Sammamish Flooding Frequently Asked Questions (FAQs)

Key point: The goal of the 1965 Sammamish River Flood Project was to provide flood protection for farm fields in the Sammamish River valley from springtime flooding. Since 1965 it has consistently met the outflow expectation from the lake to the river after March 1 of each year. The standards used for design did not address 100% of lakeshore flooding in winter.

Technical detail: Congress authorized the 1965 Sammamish River Flood Project to protect farmland from flooding in the spring (March 1 and later) and retain a minimum summer water level for the lake. The project was designed to contain spring floods that had a 40 percent probability of exceeding the spring flood of record at the time the project was built. That same flow had a 10 percent probability of exceeding the winter flood of record at the time the project was built. This means that flood control was designed to be effective 90 percent of the time in winter and that some tolerance for lakeshore flooding in the winter was expected.

The performance standard is that the lake maintain a water level of 29 feet NGVD29 (National Geodetic Vertical Datum of 1929) or lower at the Sammamish Lake Near Redmond gage (USGS 12122000) after March 1 when there is 1,500 cubic feet per second (cfs) of combined flow in the river downstream of the Bear Creek confluence. In some years, and always prior to March 1, the Sammamish River downstream of the Bear Creek confluence conveys less than 1,500 cfs when the lake elevation is at or above 29 feet NGVD.

Key point: There currently is no ability to actively control Lake Sammamish’s water level at the Sammamish River weir and no other emergency actions to lower the lake’s water level in real time.

Technical detail: The weir is a concrete structure that does not move up or down. Annual maintenance mowing of the 1,400-foot Transition Zone removes obstructions to flow below the weir.

Key point: The Ballard Locks are managed to maintain Lake Washington water levels and have little to no impact on Lake Sammamish water levels.

Technical detail: The Ballard Locks are operated by the U.S. Army Corps of Engineers to maintain Lake Washington water levels at a target elevation of about 15 feet NGVD29 in the summer and about 13 feet NGVD29 in the winter. During the Feb. 7-9, 2020 flood event, the Locks experienced the highest water inflow event (15,000-16,000 cfs) since detailed record-keeping began in 1946. In an unusual occurrence, all six spillways at the Locks were opened for this event, as there are no flood targets or concerns downstream of the Locks.

Key point: Prepare for a flood by making an emergency plan for your household, signing up to receive flood warnings and alerts for Issaquah Creek, buying flood insurance, and readying your home and property to minimize damage. You can also subscribe to USGS Water Alert for the 12122000 Sammamish Lake near Redmond gage to be notified when the lake level exceeds a particular stage.

Sandbags can be effective at preventing or reducing flood damage in some situations. Free sandbag materials are available at Marymoor Park in Redmond. Visit the Sandbag Distribution Locations webpage to learn more and see other locations where sandbag materials are available.

Learn about other important steps you can take before, during, and after a flood at kingcounty.gov/PrepareForFlooding.

Key point: The Sammamish River Transition Zone (TZ) pictured above is an area designed to control spring and summer outflows from the lake.

Technical detail: The TZ includes a weir (a low dam built to raise the level of water upstream) and a steep 1,400-foot section immediately downstream of the weir that provides the hydraulic control for the lake at higher flows. Maintaining the TZ is an important part of managing flows out of Lake Sammamish.

Key point: King County controls vegetation in the Transition Zone to ensure it functions as expected and is done according to permit conditions since signing the King County DNRP Lake Sammamish Flood Reduction Plan, April 2011.

Technical detail: Transition Zone maintenance requires permitting on a five-year cycle and includes activities to clear obstructions to Lake Sammamish outflow to meet the project’s flood performance standards. The work items for annual TZ maintenance are identical each year and are scheduled as follows:

• Manually cutting willow trees with loppers both inside the navigation channel and on the landward side of the willow buffer, and on both sides of the river. Cutting occurs every August and usually takes a day to complete.
• Mowing the high-flow channel on the left and right banks, from the edge of the willow buffer to the top of the bank. If the floodplain is dry enough, mowing also occurs under the willow branches, where accessible. Mowing occurs in September to early October and usually takes one to two weeks to complete.
• Removing and disposing of all willow trimmings and grass clippings. Disposal occurs in September to early October and usually takes two to three days.

Key point: The lake’s water level during a storm event is impacted by high inflows to the lake from creeks. High inflows to the Sammamish River from creeks below the Transition Zone can slow drainage of Lake Sammamish.

Technical detail: Lake water levels during a storm event are determined by a variety of factors:

• Water level prior to the storm event, particularly later in the winter when the water level and Sammamish River are both already higher than average due to previous storms.
• The amount and timing of high inflows from Issaquah and other creeks around the lake.
• The amount and timing of high flows from Bear Creek, which frequently causes water to backup, upstream through the transition zone, thereby decreasing lake outflow.

Key point: Due to a very wet winter, the water level on Lake Sammamish was already high prior to the February 2020 storm event. Heavy rainfall during the storm event caused high inflows to the lake, as well as a backwater effect from Bear Creek, just downstream of the lake outlet, that limited lake drainage.

Technical detail: During the February 2020 flood event, the combined results of the following three factors resulted in unusually high water levels on Lake Sammamish:

• Sustained rainfall in early February resulted in a relatively high starting water level on Lake Sammamish at the onset of the event.

• Inflows to the lake from Issaquah Creek and other creeks draining into the lake were the highest recorded since 1996.

• Persistent and high intensity rainfall within the Bear Creek watershed resulted in sustained high flows in Bear Creek (greater than 300 cfs), which caused water to backup in the TZ, reducing the amount of water that could flow out of the lake for a period of about ten days.

Key point: No, but it came close. The February 2020 flood was the second highest water level on Lake Sammamish since the Flood Project was constructed in 1965.

Technical detail: During the February 2020 flood, the lake reached a peak water level of 31.17 feet NGVD29, which was the second highest water level since construction of the Sammamish River Flood Project in 1965. It was the 15th highest water level recorded at the Sammamish Lake Near Redmond gage since record keeping began in 1939. The flood of record for this gage occurred on Feb. 12, 1951, when a water level of 33.44 NGVD29 was observed. For dates prior to October 2007, lake water level at this gage was reported only as the daily average, whereas after October 2007 the reporting interval changed to every 15 minutes, therefore a mathematical conversion is required to directly compare gage data between these time periods.

Key point: The 100-year water level on Lake Sammamish is predicted to be 33.00 feet NGVD29 or a little less than two feet higher than the February 2020 event.

Technical detail: The FEMA 100-year water level is 33.00 feet NGVD29, and there is a 1 percent chance of this occurring annually. The peak water level on Lake Sammamish during the February 2020 event was 31.17 feet NGVD29 recorded at the Sammamish Lake Near Redmond gage (USGS 12122000). Based on the full period of record for the gage, the recent peak level has about a 2 percent chance of occurring annually (equivalent to a 50-year recurrence interval).

Key point: Rainfall events and associated stream and lake levels vary from year to year. Based on current data, the February 2020 event has only a 2 percent chance of occurring every year. Climate change is predicted to increase stream runoff into the lake in the future.

Technical detail: Climate models have been used to predict future stream flows in King County watersheds, including those contributing to Lake Sammamish. A 2019 study completed by King County (Willowmoor Floodplain Restoration Project, Hydraulic Modeling Technical Memorandum) found that, by the middle of this century, the average annual flows to Lake Sammamish are anticipated to increase by 3 to 4 percent, while maximum flows will increase on the order of 3 to 7 percent. While all models are imperfect, they all point to future conditions with higher winter flows and lower spring flows.

Key point: King County has completed a watershed-scale stormwater planning effort for Bear Creeks in compliance with our National Pollutant Discharge Elimination System Phase I Municipal Stormwater Permit. King County has made significant investments in acquiring and restoring wetland and stream habitat within the Bear Creek basin to provide multiple benefits, including improved stormwater management.

Technical detail: King County is working with landowners and other jurisdictions in the Bear Creek watershed to reduce the impacts of uncontrolled stormwater runoff from developed land as identified in the Bear Creek Watershed Management Study. Impacts from uncontrolled stormwater runoff include stream flooding that is harmful to salmon, infrastructure, and homes; loss of fish habitat features and food sources due to stream channel erosion and sedimentation; and discharges of polluted runoff detrimental to water quality.

To reduce these impacts, a variety of investments and actions are being pursued and implemented. A key action among these are “stormwater retrofits,” which are improvements intended to increase control of stormwater runoff quantity and quality from thousands of acres of older developed land that was built before stormwater controls were required on new development. Additionally, other ongoing programs will:
Ensure that hundreds of existing stormwater control facilities in the watershed are regularly inspected and maintained.
Periodically inspect existing businesses in the watershed to ensure they are implementing source control best management practices to prevent stormwater pollution.
Perform watershed-wide sampling and stormwater conveyance system inspection to detect, trace, and eliminate sources of stormwater pollution.
Provide education and outreach to residents, landowners, and contractors on what they can do to reduce stormwater impacts.

Key point: King County does not have an obligation or the regulatory authority or mandate to manage Lake Sammamish to an Ordinary High Water level.

Technical detail: The regulatory authority to approve determinations of Ordinary High Water (OHW) under the Clean Water Act in Washington State is held by U.S. Army Corps of Engineers (Corps of Engineers) and Washington Department of Ecology (Ecology). The Corps of Engineers also has jurisdiction over OHW under Section 10 of the Rivers and Harbors Act as it relates to navigability of commercial waterways. The elevation value of OHW may be different for the Clean Water Act versus Section 10 of the Rivers and Harbors Act. The Section 10 OHW level is a regulatory value defined by the Corps of Engineers for their permitting process related to navigability. Section 10 OHW is a single value that is applied uniformly across a body of water. The Corps of Engineer’s OHW value of 27 feet NGVD29 has not changed since 1965.

In contrast, the Ordinary High Water Mark (OHWM), as defined by Ecology, is a point on the land that varies from place to place and can move up or down in elevation over time. In application for Shoreline Master Program Permits, cities around the lake including Bellevue, Issaquah, and Sammamish have determined the local OHWM to be somewhere between 28.0 and 28.2 feet NGVD29.

OHW and OHWM are both typically determined based on a “weight of evidence” approach using field-based observations of indicators such as water stains, organic debris lines, and the presence of wetland vegetation that indicate the zone where high water is typically found. Mature wetland vegetation is particularly important for determining OHWM because wetland plants have specific tolerances for minimum and maximum soil moisture and flooding duration and frequency. Wetland plants may tolerate a few seasons of changed water levels but cannot do so for decades. The discrepancy between the Corps of Engineers’ OHW of 27 feet NGVD29 and Ecology’s OHWM of 28.0-28.2 feet may be due to water level changes related to development or to improved scientific evaluation processes, or both.

Additional information regarding the history and challenges of OHW and OHWM on Lake Sammamish can be found in the Willomoor Floodplain Restoration Project Frequently Asked Questions.

Key point: The United States Geological Service (USGS) and King County use physical stream surveys to make necessary adjustments to the previous winter’s streamflow records.

Gauge data is provisional because stream beds change over time, requiring that the elevation of the stream bed and water surface at the gauge be surveyed and the volume of flow measured. These quality control measures ensure accurate records that can be compared year to year. Provisional data must be verified before being used for decision-making.

For more information about streamgauging, see the USGS Water Science School.

Technical detail: Publicly available, real-time hydrologic data (water flow data such as streamflow, lake water levels, etc.) are reviewed periodically to ensure accuracy. Each elevation data point recorded at a gaging station is considered provisional (subject to change) until the data is reviewed or edited and then published by the agency responsible for its collection. Generally, data is published within six months of the end of the water year (September 30). Data may be subject to significant change and is not citable until the review process is complete. Upon review, hydrologic data may be adjusted based on a variety of conditions that affect the quality of the data recordings, such as backwater due to debris and other obstructions, algal and aquatic growth, sediment movement, and general malfunction of the recording equipment.

Data users are cautioned to carefully consider the provisional nature of the information before using it for decisions that concern personal or public safety, or the conduct of business that involves substantial monetary or operational consequences. Information concerning the accuracy and appropriate uses of this data or concerning other hydrologic data may be obtained by contacting the responsible agency.

Key point: Rating curves describe the relationship between stream elevation and stream flow. Rating curves must be updated at least every year based on changes to the channel shape.

Technical detail: Development of a streamflow record requires transformation of measurements of water height (i.e. “stage,” typically reported in units of feet) into volume per unit time (i.e. “discharge,” typically reported in units of cubic feet per second). To estimate the discharge at a given stream gaging station, hydrographers –scientists who study the management of water sources– need to establish a stage-discharge relationship, and this is called a rating curve.

Rating curves are dependent on the hydraulic characteristics of the stream channel and floodplain, both at the monitoring site and downstream. Changes to the hydraulic characteristics could be minor, such as seasonal growth of aquatic vegetation or major, such as catastrophic changes due to floods. These changes may require only minor or temporary adjustments to streamflow records or could require a complete reevaluation of the rating curve.

Downstream channel hydraulic conditions, such as backwater due to tributary inflows, may also affect the rating curve and associated estimate of discharge. Sites influenced by these conditions may require a “rating shift” whereby the rating curve is shifted up or down to account for these impacts on the hydraulics.