The watershed begins at Red Box Saddle in the Angeles National Forest near Mount Wilson in the San Gabriel Mountains. As it enters the urbanized area of the watershed, the Arroyo Seco stream flows between La Cañada Flintridge on the west and Altadena on the east. Just below Devil's Gate Dam, the stream passes underneath the Foothill Freeway. At the north end of Brookside Golf Course the stream becomes channelized into a flood control channel and proceeds southward through the golf course.