 Although historically it was a smaller tributary, due to present-day flood control purposes, the channel of Peters Canyon is wider than the channel of San Diego Creek when they join.At the confluence with Peters Canyon Creek, San Diego Creek turns abruptly southwest, flowing underneath Interstate 405 for the second time.After flowing beneath the highway, San Diego Creek enters a series of slackwater pools, known as the San Joaquin Marsh, in the San Joaquin Wildlife Sanctuary.