Middle Fork Snoqualmie River the Middle Fork of the Snoqualmie River Is the Largest of the Three Forks, with a Drainage Area of 170 Square Miles

AN OVERVIEW OF THE GEOLOGY AND GEOMORPHOLOGY OF THE SNOQUALMIE RIVER WATERSHED

6.1.2. Middle Fork Snoqualmie River The Middle Fork of the Snoqualmie River is the largest of the three forks, with a drainage area of 170 square miles. The Middle Fork includes Mount Hinman, the highest point in the Snoqualmie River watershed at 7492 feet (2283 meters). In the Middle Fork watershed, about 47 percent of the land is located within the Alpine Lakes Wilderness area. Outside of the Wilderness Area, land use in the watershed consists primarily of forest production and recreation on land with mixed public/private ownership.

The upper 15 miles of the Middle Fork flows in a narrow bedrock-walled channel. The gradient ranges from 5 percent at the upstream end of this reach to 3 percent at the downstream end. The channel is relatively straight in map view and has a braided morphology. This is probably a result of a high rate of sediment delivery from numerous small, steep tributaries that drain the precipitous valley walls.

At a point approximately 15 miles (24 kilometers) downstream from the Cascade Crest and 20 miles (32 kilometers) upstream of the Three Forks confluence, the river character changes dramatically. The valley widens and the river gradient drops to approximately 0.5 percent. The channel meanders across the relatively broad valley bottom. Through this reach, the channel is relatively unconfined and exhibits pool- riffle morphology. Both the Pratt River and the Taylor River are tributaries to the Middle Fork in this reach.

For its last 5.5 miles (8.9 kilometers), the Middle Fork flows across the merged alluvial floodplain formed by the confluence of the Three Forks. The river maintains a 0.5 percent gradient and meandering plan form. This is the only section of the river with significant adjacent development, and there the banks have been locally armored and/or leveed in order to protect adjacent properties (Figure 19). The number of relict channel remnants increases approaching the confluence area, suggesting that this has been an area of active sediment deposition (Figure 20).

6.1.3. South Fork Snoqualmie River The South Fork of the Snoqualmie River has a tributary area of 85 square miles (220 square kilometers) and therefore is the smallest of the three forks. Interstate 90 follows the South Fork from North Bend for 24 miles (39 kilometers) until the highway leaves the river to cross Snoqualmie Pass. Two small dams on the South Fork take advantage of natural waterfalls to provide hydraulic head for hydroelectric production. These are located at Weeks Falls at river mile 13 (river kilometer 21) and at Twin Falls at river mile 10 (river kilometer 16). The Weeks Falls dam incorporates an inflatable bladder that forms the crest of the dam. The bladder is deflated during high flows to allow bedload to pass through the dam site. The lowest section of the South Fork flows through the city of North Bend. The river is confined between constructed levees for virtually this entire section (Figure 19).

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River Facility

Watershed Boundary Department of Natural Resources and Parks Water and Land Resources Division Stream The information included on this map has been compiled from a variety of sources and is subject to change without notice. King County makes no representations or warranties, express or implied, as to accuracy, completeness, timeliness, or rights to the use of such information. King County shall not be liable for any general, special, indirect, incidental, or consequential damages including, but not limited to, lost revenues or lost profits Lake/River resulting from the use or misuse of the information contained on this map. Any sale of this map or information on this map is prohibited except by written permission of King County.

Data Sources: King County Datasets

Incorporated Areas File Name: 0312snoGEOLriverFACIL.ai wgab Prepared by: King County DNRP/WLR GIS and Visual Communications & Web Unit