Semiquantitative X-ray diffraction analysis was performed on the clay fraction (<2 mu m) of riverbed samples from the Santa Ana River Drainage Basin, California. Bedrock sources were also analyzed. Results show that addition from older sedimentary deposits is more important to riverbed composition than is weathering in transit. Major minerals in the riverbed are montmorillonite, mica, plagioclase, K-feldspar and quartz. Present in lesser amounts are kaolinite, chlorite, calcite, amphibole and gypsum. These minerals are all present in the bedrock samples, in highly variable amounts. The composition of the coastal riverbed sediments is similar to that of marine sediments near the mouth of the river, possibly indicating recent marine transgressions in the coastal area.