We analyze the streamflow response of Sespe Creek, CA, to several large earthquakes. We find that flow increased after three earthquakes, and that the observed changes in flow have the same character. Both those earthquakes that induced static extension and those that induced static contraction cause flow to increase; streamflow thus appears to respond to dynamic strain. We find that all postseismic responses can be explained by a model in which pore pressure increases coseismically without any changes in hydraulic diffusivity. There is a particle velocity threshold in the range of 5--20 cm/s to induce the pore pressure increase.