Before entering the ocean, the river turns abruptly north near the triple junction of the Gorda, North American, and Pacific plates. This bend denotes the usual upper limit of the estuary, although brackish waters can extend as far upstream as Highway 101 during king tides. Although small, this estuary provides nursery habitat for juvenile rockfish (Sebastes melanops, S. rastrelliger) and several species of flatfish, including starry flounder ("Platichthys stellatus"), English sole ("Pleuronectes vetulus"), and speckled sanddab ("Citharichthys stigmaeus"). The estuary also serves as a migration corridor for salmonids and Pacific lamprey ("Lampetra tridentata"), and as a summer feeding ground for several marine species, including topsmelt ("Atherinops affinis") and surf smelt ("Hypomesus pretiosis"). Pacific staghorn sculpin ("Leptocottus armatus") "prickly sculpin (Cottus asper), coast range sculpin (C. aleuticus)," and Three-spine stickleback ("Gasterosteous aculeatus") are present in the estuary year-round. Above the estuary, the Mad River is home to resident coastal cutthroat trout ("O. clarki clarki"), rainbow trout ("O. mykiss irideus"), Sacramento sucker ("Catostomas occidentalis"), and Humboldt sucker ("Catostomas occidentalis humboldtianus").