Antoine Creek is a perennial tributary in the Okanogan River Basin that has supported summer steelhead. It is a small tributary with the lower reaches primarily bordered by fruit orchards. The Antoine Creek watershed has an annual surface flow of about 6 CFS and ground water production has been reported at 100 to 400 gallons per minute for wells less than 50 feet deep (Walters 1974). Antoine Creek has a drainage area of about 73 square miles at the location of the culvert replacement project. A Washington Department of Ecology peak stage gage is present 0.15 miles downstream of this project with a discontinuous period of record extending from 2002 to present. To estimate a peak discharge for Antoine Creek Pacific Hydraulic Engineers and Scientists compared the existing gauge record with a slightly longer gage record for Omak Creek which similarly drains the western slopes of the mountains on the Colville Indian Reservation and which has a drainage area of about 134 square miles. The bottomless arch culvert has the capacity to pass the 100 year flood event. This capacity will reduce the risk of this road crossing from washing out and delivering large volumes of sediment to established spawning habitat for summer steelhead.