Figure 52. Map of salt marsh flats in the Big River Estuary in 1981 from Marcus and Reneau.
The mouth of the Big River remains open year-round and forms a large estuary. Unlike many estuaries, Big River Estuary is not lagoonal but instead has a long linear channel. During high spring tides, the tidal influence extends up to 8.3 miles upstream, while during the winter tides extend 3 miles upstream (GMA 2001a). Crescent-shaped tidal flats alternate on either side of the channel corresponding with the alluvial deposits of the river (Marcus and Reneau 1981).
Wetlands in the lower reaches of Coastal Subbasin tributaries suggest that the estuary may have extended further up-river in the past (GMA 2001a).
A coastal wetland survey of the estuary was completed by CDFG in 1978 (Dana). The study found the estuary consisted of approximately 106 acres under marine water at mean low tide and approximately 191 acres of marsh and mud flats. Associated marshes are salt (63 acres), brackish (33 acres), and freshwater (59 acres). There were about 15 acres of mudflats and 2 acres of eelgrass beds (Zostera). Hypersaline ponds that dry up occasionally during the summer were intersperced throughout the salt marshes. Riparian vegetation was permanent and consisted mainly of alders and willows (Dana 1978). A series of eight salt marsh flats border the lower three miles of the estuary (Figure 52, Marcus and Reneau 1981).
Upstream from the mouth, the floodplain narrows abruptly and adjacent slopes are vegetated with second growth redwood and mixed conifer forest. The hillslopes on either side of the river are steep, with occasional remnants of the river’s former floodplain with stands of alder.
About eight miles inland the river is joined by Laguna Creek, where a large freshwater lagoon exists. This wetland is surrounded by freshwater marsh containing rushes (Juncus), cattails (Typha), and cowlily (Nuphar). Tidal influence in the summer extends as far as 8.5 miles upstream with ranges of two to four miles in the winter. Rockweed (Fucus), marine algae, has been found as far as four miles upriver (Dana 1978). More information about vegetation in and along the estuary is discussed in the Riparian Conditions section of this subbasin.
Many animal species are found in the estuary in addition to the salmonids that utilize the estuary as a migration corridor and as a nursery area for juveniles. Species of importance to fisherman include Dungeness crab, surfperch, flatfish, and surf smelt. Soft shell clams occur in intertidal flats. Old pilings in the estuary are covered in tunicates, nudibranches, barnacles, and mussels. Opossum shrimp (Neomysis) are found in the estuary and freshwater mollusks (Goniobasis) are found in the river (Dana 1978). More information about fish in the estuary is discussed in the Fish History and Status section of this subbasin.
Mammals found in the estuary include river otter, deer, mink, sea lions, and harbor seals. Sandflats at the mouth of Big River are used as rest areas by shorebirds that feed on invertebrates in the mudflats. Diving ducks, dabbling ducks, and black brant are also found. Virginia and sora rail are expected to occur in marsh areas. Woodduck nests were located in the Laguna Creek marsh in 1978, but no use was reported (Dana 1978). Twenty-four additional woodduck boxes were installed in Dry Dock Gulch and Laguna Creek in December 2002 and January 2003 (SONAR 2003). Boxes were checked for nesting success between March 11, and May 29, 2003. Two boxes out of nine were used at Dry Dock Gulch and eleven out of 15 were used at Laguna Gulch (Kight and Waldman 2003). Other birds observed in the estuary include uncommon pileated woodpeckers, osprey, great blue herons, and spotted owls (Dana 1978).