THORNTON CREEK DRAINAGE
The Thornton Creek drainage network is influenced by the topography of the land and modern infrastructure, such as drainage outflows and roads. The 7,263 acre Thornton creek watershed can be generally divided into three main sub-watersheds, referred to as the North, South and Main branches (see sub-basin map). The North branch covers land in both the cities of Seattle and Shoreline. It drains 4,446 acres and can be further divided into the sub-watersheds of its tributaries, Littles Creek and Little Brook. The headwaters of the North Branch originate in Shoreline's Ronald Bog, (N 175th St and Meridian Ave N). The pond located at the area of the bog is not a natural feature, but was created during a peat mining period in the 1950's. From here, the water flows in a southerly direction to Twin Ponds (Corliss Ave N and N 155th St.). From Twin Ponds, the creek passes through a detention pond and then into a 72-inch pipe which takes the water under I-5. From the pipe, the water surfaces into a section known as Jones Creek which passes through Jackson Park Golf Course, a wetland, Thornton Creek Park, and a condominium complex, before joining the Littles Creek tributary at NE 130th St. and !0th Ave NE.
Littles Creek flows through a residential area south along 12th Ave on the east side of I-5. It passes through Paramount Park, and its associated wetland areas and then crosses under NE 145th St. before flowing through the Jackson Park golf course. Afterwards the creek travels though commercial areas before it enters a 30-inch pipe along 10th Ave NE.
The North Branch next flows through a residential area for one and half miles where it is joined by the Little Brook tributary.
Little Brook begins near 145th St. where it flows south through a series of open water, pipes, and culverts as it travels through the Lake City Business district. It next enters a storm water detention pond, and then travels through a steep ravine area before joining the North Branch, which travels several more blocks until it joins the South Branch.
The South Branch, also known as Maple Leaf Creek drains 2,333 acres within the city of Seattle, and includes the sub-watersheds of the Victory Creek, Willow Creek, and Kramer Creek tributaries. Its headwaters originate west of I-5 near the Evergreen Cemetery, Seattle North Precinct Police Station and North Seattle Community College. Crossing under I-5 the piped creek water is fed more water by another small tributary, as well as considerable runoff associated with the Northgate shopping mall. The water emerges from the pipe near NE 103rd ST and 5th Ave NE, and then flows through Thornton Creek parks #6 and #2. Within this stretch the creek is fed water from numerous storm drains, and is joined by the waters of Victory Creek. The creek flows southeast through a residential area, being joined by Willow creek, and finally by the North Branch at NE 107th St and 35th Ave NE.
The Main Branch, which drains an additional 488 acres after the convergence of the North and South, flows into the Meadowbrook Detention pond. The pond is located at the site of the former Lake City Sewage Treatment Plant. An important change in the historic drainage patterns of Thornton Creek is found in a 72-90 inch diameter pipe located at Meadowbrook that diverts high flow water directly to Lake Washington. The remainder of the water flows toward the lake and is joined by the waters of Mock Creek and Maple Creek. Finally at Matthews Beach, nearly all of the water draining from the 7,263 acres of the Thornton Creek watershed, enters into Lake Washington.
