Seasonal And Diurnal Fluctuations Of Coarse Particulate Organic Matter Transport In A Snowmelt-Dominated Stream

We measured coarse particulate organic matter (CPOM) transport along a wood-rich, pool-riffle mountain stream in the Southern Rockies of Colorado, USA, to examine how spatial variations in storage features and temporal variations in discharge influence the transport of CPOM. Ecologists have found that the majority of annual CPOM export occurs during periods of high discharge. More recently, geomorphologists have begun to examine the transport of CPOM as bedload. There has been, however, little direct sampling of CPOM to evaluate how shorter (diurnal) and longer (seasonal peak flow) variations in discharge affect CPOM transport, and no examination of where CPOM is transported in the water column (primarily in suspension or as bedload). We collected CPOM moving as bedload, in suspension (at 0.6 of the flow depth) and at the surface to evaluate CPOM transport processes. Samples were collected at three sites: (1) in the backwater pool upstream from a channel-spanning logjam; (2) immediately downstream from the logjam; and (3) in a riffle about 10 bankfull-channel-widths downstream from any channel-spanning logjams. During sample sets, we collected samples over 15-min increments at approximately 4-hr intervals over a 24-hr period. Seven sample sets were distributed over a period of 2 months that spanned the rise, peak, and recession of the annual snowmelt flood. We found that the majority of CPOM is transported in suspension following a clockwise hysteresis loop in which CPOM peaks prior to discharge during the seasonal hydrograph.