Racehorse Creek (sir20095170.pdf) The Racehorse Creek subbasin is about 11 mi2 in size, and is located in the northeastern part of the Nooksack River basin on the northern slope of Slide Mountain in the North Fork Nooksack River basin. This mountain stream subbasin is carved through a mantle of glacial sediments and into bedrock composed of massive sandstone, conglomerate, and shale (Easterbrook and others, 2007).