LONG RIVER PROFILES, TECTONISM, AND EUSTASY - A GUIDE TO INTERPRETINGFLUXIAL TERRACES

Along three rivers at the Mendocino triple junction, northern Californ ia, strath, cut, and fill terraces have formed in response to tectonic and eustatic processes. Detailed surveying and radiometric dating at multiple sites indicate that lower reaches of the rivers are dominated by the effects of oscillating sea level, primarily aggradation and fo rmation of fill terraces during sea level high stands, alternating wit h deep incision during low stands. A eustasy-driven depositional wedge extends tens of kilometers upstream on all rivers (tapering to zero t hickness). This distance is greater than expected from studies of the effects of check dams on much smaller streams elsewhere, due in part t o the large size of these rivers. However, the change in gradient is n early identical to other base level rise studies: the depositional gra dient is about half that of the original channel. Middle to upper reac hes of each fiver are dominated by the effects of long-term uplift, pr imarily lateral and vertical erosion and formation of steep, unpaired strath terraces exposed only upstream of the depositional wedge. Verti cal incision at a rate similar to that of uplift has occurred even dur ing the present sea level high stand along rivers with highest uplift rates. Strath terraces have steeper gradients than the modern channel bed and do not merge with marine terraces at the river mouth; conseque ntly, they cannot be used to deter-mine altitudes of sea level high st ands. Strath formation is a continuous process of response to long-ter m uplift, and its occurrence varies spatially along a river depending on stream power, and hence position, upstream. Strath terraces are fou nd only along certain parts of a coastal stream: upstream of the aggra dational effects of oscillating sea level, and far enough downstream t hat stream power is in excess of that needed to transport the prevaili ng sediment load. For a given size river, the greater the uplift rate, the greater the rate of vertical incision and, consequently, the less the likelihood of strath terrace formation and preservation.