Castle Creek is a tributary of the North Fork Toutle River on the flank of Mount St. Helens in Washington state. It rises about two miles (three kilometers) northwest of the crater rim () and flows generally to the northwest. The outflow of Castle Lake joins the creek at the lake's north end () after traveling a few hundred meters on the South Fork Castle Creek.The National Map, United States Geological Survey, accessed 2018-07-13 It joins the North Fork Toutle River at , elevation 2,200 feet.
Castle Lake was created by an avalanche subsequent to the May 18, 1980 eruption of Mount St. Helens which blocked South Fork Castle Creek. Pre-1980 maps show this area as "Castle Creek Marsh".United States Geological Survey topographic "Elk Rock" 15-minute quadrangle, 1964 ed. marked historical with comment "Feature is now part of Castle Lake, formed by a dam created resulting from the eruption of Mount Saint Helens in 1980."
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