Parts of this subwatershed are unprotected by levees and are vulnerable to 100-year floods. Historically Fairview Creek flowed north into the Columbia River through a wetlands slightly upstream of Big Four Corners. In the early 20th century, water managers dug an artificial channel connecting the Fairview Creek wetlands to the slough. In 1960, they built a dam on the west side of the wetlands to create Fairview Lake for water storage and recreation. It covers about and is deep. Fairview Creek forms in a wetland near Grant Butte and flows north for through the cities of Gresham and Fairview to reach the lake. Fairview Creek has two named tributaries, No Name Creek, and Clear Creek. A smaller stream, Osborn Creek, also flows into the lake, which empties through a weir and culvert system into the upper slough. With one exception, the streams feeding Fairview Lake are the watershed's only remaining creeks, although springs also reach the surface. Wilkes Creek, the slough's only free flowing tributary, is about long and enters the upper slough from the south. Dozens of similar streams that once flowed into the slough from the south have all been piped or filled. Many bodies of water in addition to the main slough channel lie within the drainage basin. The area around the middle slough contains several slough arms and small lakes, including Buffalo Slough, Whitaker Slough, Johnson Lake, Whitaker Ponds, and Prison Pond. In the lower slough, Smith and Bybee Wetlands Natural Area at is one of the largest urban freshwater wetlands in the United States. A side slough called North Slough connects Bybee Lake and the main slough channel. A water control structure at the outlet from Bybee Lake to the North Slough regulates the lakes' levels. Groundwater discharges from an aquifer near the surface supply an estimated flow between to the middle and upper sloughs. Rain infiltration and artificial sumps recharge this shallow aquifer. Geologists have identified four other major aquifers separated by relatively impermeable clays or other strata at various levels below the surface aquifer. The City of Portland's water bureau manages the Columbia South Shore Well Field that taps the deeper aquifers. As of 2004, 25 active wells drawing from depths ranging from could produce up to a day from the field. These were drilled between 1976 and 2003 to supplement the city's main water supply from the Bull Run Watershed during droughts or emergencies.