Elwha Watershed Information Resource

Currently, the Elwha-Dungeness Watershed is considered a fish critical basin. Fish critical basins are watershed areas where low flows are considered limiting factors for salmon populations. The Elwha River is located in one of the eight critical basins in western Washington. Water is available for people in the community (City of Port Angeles and the Lower Elwha Klallam Tribe) to use for drinking water and their daily activities (such as cooking and laundry). However, water use is monitored through a volunteer water acquisition program where farmers, ranchers, and other water right holders can sell, lease, or donate their water rights to help conserve water in critically low areas for fish.

The two dams on the river have altered the flow of the natural river and have reduced flows in the lower river. Dam removal is not expected to alter the water quantity, except for a few months of high flows during removal and the moderation of flood flows for 2-3 months during construction of a diversion channel at Elwha Dam (NPS 1996). Additionally, the amount of groundwater in some wells near the river may be altered for a short period due to increases in fine sediments in the aquifer.

The Elwha River is the fourth largest river (by drainage area) in the Olympic Peninsula and 83 percent of the Elwha watershed (pop-up jpg) lies within the Olympic National Park (NPS 1996). The mainstem of the Elwha River is about 45 miles long with 100 miles of tributaries. In the Upper Elwha watershed (the portion that lies within the park), the headwaters (elevation between 2100 and 2500 feet) of the Elwha River begin in the Olympic Mountains (Elwha-Dungeness Planning Unit 2005). The Lower Elwha watershed begins at the park boundary (river mile 9.7) and ends at the river’s mouth.

Rain and melting snow both contribute significantly to the quantity of water in the Elwha River. Small levels of flooding in the river are common, but about once a decade the area gets enough rain and/or melting snow to cause disastrous flooding.

Rain

Elwha River high flow rate. National Park Service

The Elwha-Dungeness watershed does not have a large annual rainfall, because it is in the rain shadow of the Olympic Mountains, unlike the western side of the Olympic Peninsula, which is the wettest area in the continental U.S., receiving as much as 160 inches of rain each year. The average annual rainfall for the northeast Olympic Peninsula (where the Elwha River is located) and San Juan Island area is between 25 and 30 inches. The 50-year average annual rainfall for Port Angeles is approximately 22 inches (U. S. Geological Survey 2000). Most of the rain in the area falls in the form of light rain and drizzle, with more measurable precipitation occurring 3-7 days each summer and 2-3 weeks each winter.

Snow

The amount of annual snowfall for the Elwha watershed varies by elevation. For example, the average snowfall per year at these locations is:

• 4.3 inches in Port Angeles (at sea level),
• 17 inches at the Elwha Ranger Station (elevation 300 ft), and
• 400 inches at Hurricane Ridge (elevation 5,240 feet).

Visit the National Water and Climate Center Website for current and historical information collected at the Waterhole SNOTEL (snow pack telemetry) site near Hurricane Ridge.

Flooding

Flooding can occur at any time of year, but flooding occurs most frequently between November and February (Washington State Hazard Mitigation Plan 2004). Fall and early winter floods tend to be the result of heavy rainfall on frozen or wet ground, whereas winter and early spring floods are typically caused by lower elevation snow melt and rainfall.

Flooding rivers on the north end of the Olympic Peninsula is common: typically occurring every 2-5 years. However, these types of common floods cause only minor local damage. Floods that cause major damage occur less frequently, with floods that meet the Presidential Disaster Declaration level occurring approximately every 8 years.

To monitor the Elwha River for possible flooding, the water level is checked every half hour from McDonald Bridge at river mile 8.6 (NOAA 2006):

• At approximately 18 feet, the Elwha will spill over its banks.
• By the time the flood stage of 20 feet is reached, water will cover nearby roads and approach some residential areas.
• At 21.5 feet, a moderate flood stage level is reached. Flood waters will move further over roads, land, and housing areas. Damage to river banks usually begins to occur at this point.
• At 23 feet, a major flood stage is reached and the water of the Elwha will be deep, fast moving, and causing damage to its banks.

Reference

Elwha-Dungeness Planning Unit. 2005. Elwha-Dungeness Watershed Plan, Water Resource Inventory Area 18 (WRIA 18) and Sequim Bay in West WRIA 17. Published by Clallam County. Volume 1: Chapters 1-3 and 15 appendices; Volume 2: Appendix 3-E.

NOAA (National Oceanic and Atmospheric Administration). 2006. National Weather Service Advanced Hydrologic Prediction Service, Seattle, WA.

National Park Service. 1996. Elwha River Ecosystem Restoration, Draft Environmental Impact Statement. National Park Service, Olympic National Park, 600 East Park Avenue, Port Angeles, Washington, 98362.

U. S. Geological Survey. 2000. An assessment of available data and information and possible methods of analysis of the watershed resources of the Elwha-Morse watershed, Washington. U.S. Geological Survey, Water Resources Division, Tacoma, WA.

Washington State Hazard Mitigation Plan. 2004. Hazard Flood Profile. Washington Military Department, Emergency Management Division.