For example, flood deposits have been found under tidal marshes around San Francisco Bay in northern California. Typically the inflowing river waters that spread across the marshes deposit only thin traces of the finest sediments—clays and silts. The more vigorous flows of major floods carry larger particles and deposit thicker and coarser layers. The flood layers can be dated using the common radiocarbon-dating method, which in this application is accurate to within about 100 years. A study of the marsh cores by one of us (Ingram) and geographer Frances Malamud-Roam revealed deposits from massive flooding around A.D. 1100, 1400 and 1650. A distinct layer from the 1861–62 event is difficult to distinguish, however, because hydraulic gold mining in the Sierra Nevada foothills during the decade before and after the flood moved enormous volumes of silt and sand that essentially wiped out whatever traces the floodwaters might have left.
Sediment cores taken from beneath San Francisco Bay itself also indicate that in 1400 the bay was filled with freshwater (as it was during the 1861–62 event), indicating a massive flood.