Randal L. Dinehart, Sacramento California
The Sacramento and San Joaquin Rivers are connected ahead of their confluence by a narrow channel, the naturally formed Threemile Slough. To follow the transport pathways of sandy bed material from the lower Sacramento River, bedforms have been mapped periodically since 1998 in a 0.5 km reach at the south end of the slough where a sand shoal forms at the entrance to the San Joaquin River. A gaging station at the slough is operated by the U.S. Geological Survey to record water stage and tidal velocity.
Various types of bedforms have been surveyed to document sand movement in the tidal environment over periods of weeks and months. Asymmetric bedforms (sand dunes) migrate throughout the surveyed reach, with heights often exceeding 3 meters, indicating high transport rates and substantial roughness effects on the flow.
Although individual dune profiles in the reach are mostly symmetrical, dunes nearer the shoal have slipfaces at their south end, indicating southward migration of bedforms. Northward-oriented bedforms at the west side of the surveyed reach usually coexist with southward-oriented bedforms at the east side. As seen in planform, crests of the largest dunes are diagonal, not perpendicular, to the channel banks. The diagonal orientation is a consequence of bidirectional migration in the reach during dry seasons.
However, bedforms across the slough were all migrating southward during high discharges from the Sacramento River in February 2000. A bedform-transport rate of 100 tonnes/day toward the San Joaquin River was calculated for the period February 18-25, 2000. Tidal flows in Threemile Slough move south into the San Joaquin River during flood tide, allowing sediment transported from the Sacramento River to become available for movement in the San Joaquin River.