Reservoir Method (Depth Dependent Deposition) 

Some depositional systems, particularly prograding reservoir deltas, depart from the veneer assumption, depositing more in the deeper parts of the channel. 

The figure below illustrates how reservoirs tend to fill the historic channels upstream in the region of delta deposition but follow more of a veneer pattern downstream, where sediment settles out from suspension.  
In the upstream reach of this reservoir (the XSs on the upper left) the delta essentially erases evidence of the channel.
(Note: This figure is very useful but we have lost the reference.  If anyone know what paper it comes from please contact the RAS inbox.) 

The Reservoir Option in HEC RAS was designed to replicate this behavior by depositing more sediment in the deeper regions of the cross section.  The algorithm increased deposition across the wetted nodes (between the movable bed limits if the method is limited to the channel) proportional to water depth.  The Reservoir deposition method is available in the channel and overbank, but it is designed to fill the channel in backwater situations.  If the Reservoir method is selected for both the channel and the overbanks, it will apply the proportional deposition approach to the entire wetted cross section. 

Differential Deposition will Generate Flat Cross-Sections in non-Monotonic Bed Change Regimes

Depth dependent deposition may seem like an attractive alternative to the veneer method in non-reservoir, riverine settings.  However, it is often unstable in systems that alternately deposit and erode.  Depth dependent deposition cross sections converge on flat channels and can presently lose the channel shape during deposition events.  This method performs best in settings of monotonic channel deposition.

Differential deposition vs Veneer in a Reservoir

Differential deposition based on depth is a function of length of reservoir, longitudinal location, and gradation of inflowing material (i.e. fall velocity).  In the upstream portion of the reservoir, where coarser materials fill the valley, the channel will fill.  The depth dependent method is appropriate here.  However, fine materials that settle out in the reservoir pool, tend to blanket the flooded valley with a fine uniform veneer, often maintaining the pre-reservoir cross section shape through decades of deposition.