Abstract
This paper examines the effect of tide‐induced flow reversals on sediment transport and channel morphology in the riverine–estuarine transition zone of the Raritan River, New Jersey, U.S.A. The Raritan (drainage area 2862 km
2
) has a mean annual discharge of 52 m
3
sec
−1
, and is about 100 km long, with the lower 26 km being tidally affected. There are four major hydrologic/morphologic zones in the lower 50 km of the river, in downstream order, as follows. Reach 1—a non‐tidal zone, with relatively steep channel slope (0·0005) and a straight, shallow channel on bedrock with a thalweg that meanders through a patchy veneer of gravel and isolated gravel bars. Reach 2—a wide, multiple‐thalweg channel with large gravel bars exposed at low tide; this reach is 4 km long, and water surface slopes vary from 0·0005 downstream on ebb flows to near 0 on flood. Reach 3—a deep, highly sinuous (S = 1·5), meandering reach about 12 km long with sandy gravel bed; velocities are bidirectional in this reach (relatively low at its upstream end, increasing downstream as the semidiurnal tidal volume increases). Reach 4—a very wide, low‐sinuosity (S = 1·1) reach about 10 km long just above the river mouth and bordered by an extensive salt marsh complex; flow in the channel is strongly reversing and sandy bed sediments are transported in both upstream and downstream directions.
Each of these reaches has characteristic patterns of sediment transport as determined by channel morphology and hydraulics. In the upper reach, high velocities cause relatively rapid downstream transport of sands and gravels, with little storage. The coarser clasts are stored in large gravel bars in Reach 2 as a result of the reduction in velocity at high tide. This reach is the geomorphological expression of the head of tide. The lower two reaches represent a tidally dominated system, in which there is a gradual transition from small flow reversals and dominance of high runoff events at the upper end to strongly reversing sediment transport and reduced influence of high runoff events at the river mouth. The high sinuosity and rapid downstream widening of the lower two reaches are characteristic of tidally dominated systems.