THE TECTONIC ORIGIN OF LAKE MERRITT

Lake Merritt is a striking example of a drowned river valley incised into the East Bay alluvial plain, an erosional anomaly in an aggrading landscape. It owes its persistence to a relatively high gradient stream network (Pleistocene Merritt Creek), nearer to the shore than neighboring streams, that drains a tilted crustal block (Piedmont block) lying on the seaward (west) side of the Hayward fault. Whereas other East Bay streams are regularly beheaded where they cross the fault, Merritt Creek’s watershed is not disrupted by fault motion. Geomorphic and geodetic evidence suggests that upift and tilting of the Piedmont block occurred about 1 Ma when transcurrent displacement carried it northward past a salient in the fault at the San Leandro Gabbro. A prominent water gap in the block, Dimond Canyon, corresponds to the position of San Leandro Creek at that time. Today the canyon holds an underfit stream, Sausal Creek. For about 1 million years, or 8–10 glacial cycles, rainfall on the Piedmont block supported an integrated stream network on resistant rock with the power and persistence to repeatedly incise deeper channels at sea-level lowstands than other East Bay streams. Lake Merritt, with its entourage of Pleistocene landforms, exists today because of events that began a million years ago.