Patterson Creek is the largest left bank tributary to the Snoqualmie River downstream of Fall City. Located along the western edge of the watershed, Patterson Creek flows in a southeasterly direction for most of its length before turning north and traversing the Snoqualmie River’s floodplain through farmland. 5.4% of the sub-basin lies within the City of Sammamish where it features medium-density urban residential land uses. In addition, roughly 5% of the sub-basin lies within an unincorporated area with a low-density residential zoning designation. Located on the southwest flank of the sub-basin, the area is home to the Members Club at Aldarra golf course and associated residential developments.

Like many other tributaries in the watershed, the lower portions of Patterson Creek are dedicated to agriculture by way of their APD designation. Outside of the agricultural area, Patterson Creek is one of the most rapidly developing sub-basins in the Snoqualmie Watershed (Haring, 2002).

The sub-basin has a broad elevation range from 70 feet above mean sea level to 1400 feet in the southwest corner of the basin (King County, 2004). As a result, the basin has many small tributaries that descend through steep ravines before reaching the valley floor. The mainstem itself originates in a broad, low-gradient catchment on an upland plateau before descending through a ravine to form the much lower gradient stream course that roughly follows State Route 202. The stream itself is very small for the size of the valley that it occupies. This is because the valley itself was carved by glaciers rather than by the stream itself. It is thought that the stream channel once served as the outlet for glacial Lake Snoqualmie (Bethel, 2004). The main channel features many lateral wetlands that extend nearly the entire length of the stream from Redmond-Fall City Road to the Snoqualmie River confluence (King County, 2004).

Salmonids make extensive use of the Patterson Creek sub-basin. Steelhead and coho salmon occupy the mainstem and several key tributaries for both spawning and rearing. Canyon Creek, a major tributary that flows through the Aldarra golf complex, is not only an important stream for coho and steelhead, but is also known to support Chinook salmon. Chinook also utilize a substantial portion of the mainstem of Patterson Creek. For purposes of state water quality standards, Patterson Creek is considered Core summer salmonid habitat.

Fish habitat conditions in the mainstem of Patterson Creek are generally regarded as poor, with riparian degradation, fish passage barriers and the lack of large wood in the stream among the biggest problems (Haring, 2002). The basin was historically largely forested, but the valley floor in particular features only a modest fraction of historic forest cover due to previous logging, agricultural and land clearing activities. The amount of active agriculture appears to have declined in the last decade or so and many areas of the mainstem floodplain appear to be undergoing a slow transition from agriculture back to their original emergent and forested wetland conditions.

Due to the prevalence of agriculture and low-density residential land uses, impervious area accounts for only 3.2% of the sub-basin, with a projected increase to 4.9% (a 53% increase) under a full build-out scenario, assuming current land-use designations (King County, 2004). This fairly low level of imperviousness suggests high potential for restoring hydrologic processes in the basin. Key actions include forest and wetland retention, floodplain reconnection and riparian restoration (King County, 2004; Snohomish Basin Salmon Recovery Forum, 2005).