Stabilization of a gravel channel by large streamside obstructions and bedrock bends, Jacoby Creek, northwestern California

Jacoby Creek (bed width = 12 m; bankfull discharge = 32.6 m³/s) contains stationary gravel bars that have forms and positions controlled by numerous large streamside obstructions (bedrock outcrops, large woody debris, and rooted bank projections) and bedrock bends. Bank-projection width and bar volume measured in 104 channel segments 1 bed-width long are significantly cross-correlated at lags of −1, 3, and 4, indicating the tendency for large obstructions and bends to form bars 3 to 4 bed-widths downstream and 1 bed-width upstream. All of the 18 bars downstream of large obstructions or bends in the study reach were along the obstruction side of the channel or outside bank of the bend. Most of the pools (85%) were next to large obstructions or in bends; conversely, 92% of large obstructions or bends had pools. Comparison of the volume of four bars with volumetric bar changes and volume of bedload transported during four high-flow events suggests that rates of sediment transport were sufficient to cause major changes in bars during bankfull events. The only important channel changes observed in 4 yr, however, have been associated with the movement of large woody debris and with changes in the angle at which the flow approaches an obstruction.

A general model is proposed that large obstructions and non-alluvial bends stabilize the form and location of gravel bars. Bars are stabilized by two related mechanisms.

1. Large obstructions and bends cause intense, quasi-steady, secondary circulation in scour holes that terminate upstream bars at fixed locations. Obstruction width, channel deflection, scour-hole width, and bed width were measured at 26 obstructions. These data show that obstructions wider than approximately one-third of the bed form “pools” spanning the entire channel and, thus, terminating bars; smaller obstructions form “scour holes” contained within a single bar.

2. Bars are deposited upstream of large obstructions and sharp bends because of backwater reductions in stream power. Bars are deposited downstream because flow energy is expended around obstructions and bends and because the flow expands downstream of constrictions that result from large obstructions.

The formation of bars and pools inherent in many gravel channels can, thus, be enhanced and fixed in position by flow structures set up around large obstructions and bends formed of resistant materials.