ROLE OF LOCAL BEDFORM STRUCTURES IN A LOW-GRADIENT, RIFFLE-POOL STREAM

Basal and critical shear stress calculations suggest local bedforms play specific roles in a stream's equilibrium in relation to discharge rates. Longitudinal profile and floodplain data of a low-gradient, riffle-pool stream near Stockton, MN, were collected using a laser-prism total station, and channel velocity profiles and discharge were measured using an Acoustic Doppler Velocimeter. ArcGIS was used to spatially analyze channel morphology and velocity profiles by creating two-dimensional, value weighted plots. Shear stresses were then calculated based on channel geometry.

Due to their larger cross-sectional areas, pools exhibit slower current velocities, and finer-grained particles armor the bed with a slight fining downstream. Silting was observed on inner pool bends during summer and fall months, which may indicate an active filling process. Pools may provide accommodation space and conditions needed to deposit fine-grained particles initially flushed out of the system during spring flood events. Water depths shallow and current velocities increase as pools transition into riffle structures.

Based on shear stress analysis, both pools and riffles are stable during baseflow conditions. This stability during baseflow is also supported by field observations; however, during bankfull conditions only riffles were calculated to be stable according to d₈₅ particle size. Pools are unstable during bankfull conditions at the upstream point of the structure, where smaller sand-sized particles dominate the armored surface. Basal shear stress within a riffle increases downstream reaching a maximum at the transition between the riffle and the adjacent downstream pool. This point of maximum basal shear stress, which is directly upstream of the unstable cross-sections, may indicate a potential of pool head-cutting during bankfull discharges. Basal and critical shear stress analysis supports riffle structures serving as energy enhancers during baseflow conditions by decreasing cross-sectional area and increasing current velocity. Available energy is then expended in pools by increasing cross-sectional area and depth. Thus, baseflow processes may provide insight to where work is being done during bankfull discharges.