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Abstract. Clinoforms are the building blocks of prograding stratigraphic sequences. These sigmoid-shaped surfaces can be found forming today on modem deltas. Sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions, with a maximum accumulation rate on the upper foreset region. We present a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river. Model clinoform surfaces are generated using an equation for the conservation of suspended sediment concentration, together with a conservation of fluid equation for simple time-averaged flow velocity fields. In the model, suspended sediment is advected horizontally into a basin, and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion. In shallow water, shear stresses are too large to allow deposition, and sediment bypasses the topset region. With increasing water depth, near-bed shear stresses decrease, and sediment is allowed to deposit at the foreset region, with gradually decreasing rates toward deeper water. This sedimentation pattern leads to progradation of the clinoform surfaces through time. The clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms. These include the gradual slope rollover at the topset and bottomset, steeper foreset slopes with increased grain size, and an increase in foreset slope through time as clinoforms prograde into deeper water. Because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems, the model is an important step toward unraveling the physical processes associated with these deposits.

Amazon Channel displays a relatively smooth, concave-up, longitudinal talweg depth profile that suggests a system in equilibrium. The gradual seaward decrease in channel slope occurs despite large variations in the gradient of the valley over which the channel is built. Equilibrium was apparently reached by adjustment of channel slope by two basic processes: changes of channel sinuosity and entrenchment/aggradation of the channel talweg. Channel equilibrium was disrupted in the past, when a knickpoint formed at a channel bifurcation site, associated with the formation of a new channel down-fan and causing a relative base-level drop. The magnitude of base-level drop increases with pre-bifurcation channel sinuosity and with aggradation of the talweg above the adjacent fan. Present-day channel morphology shows that knickpoints would be most pronounced for a bifurcation occurring on the middle fan, and less pronounced on the upper and lower fans. The present longitudinal profile indicates that past knickpoints have largely been erased from the profile. The mechanisms associated with channel equilibrium involved sudden sinuosity changes and channel entrenchment upstream of the bifurcation site, and marked aggradation downstream as a new channel levee formed. Channel bifurcation is related to periods of enhanced channel progradation interpreted to result from increased influx of terrigenous sediment to the fan.

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%-142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of fl ow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform.However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The fi rst bend served as a fi lter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this fi ltering bend and was lost to the overbank surface.