Freshwater Aquatic and Riparian ecosystems are defined as the intermediary zones that occur between fully terrestrial areas and watercourses or water bodies. These ecosystems share a set of unique qualities due to their soil and vegetation characteristics, which are strongly impacted by the free or unbound water present in the earth.
There are 28 distinct drainage basins in the Santa Monica Bay watershed, encompassing creeks, streams, and depressional freshwater wetlands—with more located in the north part of the Bay watershed than the south.
At one time, the Santa Monica Bay watershed was covered with a web of creeks, streams, and depressional freshwater wetlands that were fed by seasonal rains and natural springs (Stein et al. 2014). Many of the natural streams in the watershed were intermittent, with the greatest flows occurring in the wet season during winter. The streams from the eastern Santa Monica Mountains and the northern part of the Palos Verdes Peninsula would flow out of the hills and onto the coastal plain, where they would meander or braid before gradually making their way to the ocean through the once-expansive Ballona Wetlands.
In the north, Malibu Creek is the largest un-channelized creek in the Bay watershed. Smaller drainage basins are present throughout the Santa Monica Mountains. Many in the eastern Santa Monica Mountains are confined to concrete channels for at least part of their lengths.
In the central Bay, the Ballona Creek drainage basin dominates. At 130 square miles, it is the largest sub-watershed draining into Santa Monica Bay. Ballona Creek drains portions of west central Los Angeles and several other cities, as well as the southeastern portion of the Santa Monica Mountains. Most of Ballona Creek was channelized in the 1930s for flood control purposes, and consequently, little riparian habitat remains.
Smaller drainage basins can be found throughout the South Bay and the Palos Verdes Peninsula. Most of these have been buried or replaced with storm drains (LA Creek Freak 2012).
In a natural state, these habitats comprise the stream or river and the stream or river banks that the water flows through or over at higher water levels. These banks are part of the flood plain, where sediment is held in place by the roots of the many types of vegetation found naturally in these areas, e.g., grasses, sedges, shrubs, and trees. When considered together, these zones slow water flows, allow for water to soak into the ground, and capture sediment and pollutants from the watershed around them, while supporting many species of animals, as listed above. In turn, healthy riparian zones supply downstream areas with water and sediments needed to maintain beaches and rocky reefs via natural patterns of erosion and transport.