Santa Ana River: An example of a sandy braided floodplain system showing sediment source area imprintation and selective sediment modification

Abstract The Santa Ana River floodplain can be divided into five geomorphic environments recognized by both morphology and sedimentary characteristics: braided channels, bars, vegetated islands, sand flats and floodplain terraces. Braided channels which vary in width from 30 to 80 m and are bordered by sand flats, flow round vegetated islands and emergent mid-channel bars. Deep braided channels have linguoid dume bedforms which are reworked during falling flood stages. If channel bars emerge in early spring, vegetation is rapidly established and they may become vegetated islands. Bars emerging in minor channels and in areas of increased channel width may develop into extensive sand flats. Away from the channel margins, sand flats are subject to wind erosion as the water table declines in summer and fall. This sediment is deposited in sand sheets and vegetated dunes. These distal sand flat environments are characterized by a well-sorted, medium sand-sized sediment population and are separated from negatively skewed, moderately sorted coarse channel sands by a mixed sediment population along channel margins. The drainage basin discharge is dominated by the regional Mediterranean climate. The coarse channel sediment reflects the large temporal variations in drainage discharge and the resulting greater variability and competence to transport sediment. Major decreases in channel width during both seasonal summer drought and prolonged periodic drought exposes large areas of the floodplain to selective wind winnowing of fluvially derived sediment. This process operates on a small scale today, but during arid conditions in the late Wisconsin interglacial interval (prior to 22,000 yrs B.P.) extensive sand flats along the Santa Ana River, distal alluvial fans, and sediment from minor tributaries were the sediment source for a major riverine dune field adjacent to the floodplain. This dune field was stabilized during the late Wisconsin glacial interval. Subsequent channel entrenchment has isolated this dune field from its source region. Santa Ana River sediments originate from two distinct geologic provinces: the San Gabriel-San Bernardino granitic and metamorphic complex to the north and the granitic Perris Block to the south. Settling-tube analysis of sediments from tributary streams draining the Perris Block shows a distinctive bimodal distribution characterized by minimal sediment in the very coarse sand size (−1 to 0 phi size) inherited from the granitic grus parent material. This is imprinted onto all sediment from this source. Transportation results in sediment erosion; sediments are better sorted and finer grained with increasing distance from their source. The sediment in these intermittent tributary streams frequently represents a single sedimentary event (minor flash flood), and all the sediment, including the fines, is left behind. This produces a negatively skewed sediment distribution when the fines are added to an existing deposit within the limits of a predetermined size range. Older alluvium is strongly negatively skewed and may indicate in-situ weathering. Coarse channel sands from the Santa Ana River maintain this characteristic sediment distribution, but selective deposition and winnowing by the wind on the sand flats creates a distinctive floodplain sub-environment. Analysis of these deposits and material from tributary streams illustrates that a particle population is dependent upon grain size distribution at the initial source. Sediment populations are modified by subsequent erosion, selective grain-size transport deposition, and total deposition and mixing of several source areas.