This former marshy inlet of Lake Washington became the slough it is today when the completion of the Hiram M. Chittenden Locks in 1917 lowered the surface of the lake 8 feet and drained the wetlands. The Mercer Slough is an outlet of Kelsey Creek and is home to Mercer Slough State Park and a small blueberry farm. The Mercer Slough Environmental Education Center offers guided canoe tours of the park from Memorial Day through Labor Day.
Launching from the very kayak-friendly Sweyolocken Boat Launch puts the I-90 overpass and Lake Washington to the south and the bulk of Mercer Slough State Park to the northwest. Heading into the popular Mercer Slough State Park, the slough flows uninterrupted for just under a mile when the path forks. Staying to the right, be sure to keep an eye out for turtles. The slough continues northbound for another half-mile where it forks again. The path to the right leads to a drainage ladder and a shallow, narrow, and dark tunnel under Lake Washington Boulevard SE, and the path to the left loops back around to the south and reconnects back through where the slough first forked. Use caution when taking the western path, as there are many logs hidden just below the surface. Following the slough south back toward the Sweyolocken Boat Launch and beyond leads under I-90 and into Lake Washington.