For more information, see Department of Ecology: River & Stream Flow Monitoring
The Black River drains southwest from the south end of Black Lake into the Chehalis River near Oakville in Grays Harbor County. The Black River drainage is approximately 144 square miles, with 105 square miles in Thurston County. In general, the Black River is a slow flowing river with a broad floodplain. Most flooding along the main stem of the river is inundation flooding with low velocity of the flood water.
The Black River drainage is divided into two uniquely different halves. The west side drains the Capitol Forest area and the main tributaries are Dempsey, Waddell, and Mima creeks. This area ranges in elevation from 2,659 feet at Capitol Peak down to approximately 200 feet at the Black River valley floor. This side is subject to high intensity, short duration rain events which can produce flash flooding in these creeks. This flooding can be compounded by snow in the watershed. In general, snow melt alone will not cause flooding in this watershed.
The east side of the watershed drains the relatively flat area south of Tumwater, west of Offutt Lake and north of Tenino. The elevation difference of this area is approximately 200 feet. The main streams draining this side are Salmon and Beaver creeks and Bloom Ditch. These are very slow flowing water systems that tend to cause inundation flooding with no velocity. This side of the drainage is susceptible to high groundwater flooding when periods of extended rain exist.
Because of its flat topography the Black River is also susceptible to flooding caused by flood waters backing up from the Chehalis River. This is especially true when flooding on the Chehalis River is concurrent with high tides along the coast.
Due to these varying flooding scenarios, we urge floodplain residents to keep an eye on the river and to continue to monitor local media reports even if the rain has stopped and all other local rivers have receded. In April 2005 the Washington State Department of Ecology established a river gauging station on the Black River where it crosses US Highway 12. Below is a link and view of that site. Unlike the gauging stations on the Chehalis at Prather Road Bridge and at Porter, this site has not been rated and is not currently modeled to forecast flood levels. However, as a planning tool by comparing this gauge to the ones on the Chehalis at Porter and Prather Road Bridge we have developed the following impacts.
At a stage of 6 feet residents should be aware that the river is likely to flood. At a stage height of 8 feet the Black River has reached Flood Stage; the river will spill out of its banks into nearby fields and woods with limited water over a few spots on local roads. At 10 feet moderate flooding will occur. This stage height corresponds to 15.5 feet at the Prather Road Bridge on the Chehalis River. At this level, the Chehalis River in Thurston County will flood several roads in Independence Valley with swiftly moving water including US Highway 12 and James, Independence, Moon and Anderson roads. Flood waters will cut off access to and from the Chehalis Reservation and inundate nearby farm lands. Some residential structures may be threatened. When the Black River reaches a stage of 12 feet, Major Flooding will occur. During the flooding in December 2007 the gauge on the Black River recorded a stage height of 14.5 feet.