River Mouths, Brakish and Estuarine Coastal Wetlands
II. Regional Patterns
A distinct moisture gradient exists from northern to southern California, and the river mouths reflect the latitudinal decline in river water. Southern river mouths close more frequently and lagoons and river channels are usually dry in summer, unless they are covered by tidal water. Some southern lagoons become hypersaline in the dry season (Zedler 1983). This geographic moisture pattern is less obvious along the MBNMS, but there are distinct geographic patterns from the northern MBNMS coast creeks and lagoons, to the Monterey Bay area and the more southerly coast.
The north coast beyond San Francisco Bay is bathed by muddy waters from the Bay and by sediment sliding from steep coastal bluffs cut into the Franciscan geologic formation, a highly erodable substrate (Moss Landing Marine Labs 1996). Just beyond the northern border of the MBNMS, Bolinas Lagoon captures two major creeks with a large sand bar deposited from longshore drift (Bascom 1964). Just beyond the southern border of the MBNMS, Morro Bay is formed in a similar manner. In contrast, however, nearly all creeks and rivers in the MBNMS dump directly into the ocean. The mouths are usually open until summer when they are blocked by onshore and longshore movement of sand beaches (Bascom 1964). Low summer flows often keep the lagoons and adjacent river channels ponded until the river flows with winter rains (Smith 1990).
Most of the creeks and lagoons along the MBNMS northern coast (from Muir Beach to Santa Cruz) are similar, particularly those in the same geographic areas (e.g. Redwood Creek, Pescadero Marsh, and Scotts Creek; Figure 1). The watersheds for essentially all of these systems have been radically modified, especially from logging most of the native forests. The Monterey Bay area includes the mouths of the largest MBNMS rivers, including the San Lorenzo, Pajaro, Salinas and Carmel Rivers (Figure 1). These are big, highly modified watersheds. At one time, the Pajaro and Salinas Rivers used the same mouth near Moss Landing (Gordon 1996). There are many small creeks south of the Bay area, mostly along the heavily forested Big Sur coast, which captures so much water that most creeks flow throughout the year.
River mouths and lagoons are periodically disturbed by highly disruptive events including floods, earthquakes, storm waves, extreme tides, and humans who dam and dike the river channel and control winter breakout. Strong river flow, tidal inputs, and sand bar closure create radically different seasonal habitats in the river mouth and lagoon (Smith 1973, Smith 1990, Zedler 1983, 1986).
The habitats within the river mouth and lagoon include the sand beach and bar, flowing river water, lagoon and channel ponds, lagoon and channel bottoms, emergent vegetation and marsh, riparian corridor and adjacent sand dunes. The river mouth and lagoon depend on the upper river watershed, connected through the wet corridors.
Section I. West Coast vs. East Coast River Mouths