Controls on Patterns of Coarse Organic Particle Retention in Headwater Streams

Organic matter retention is an integral ecosystem process affecting C and nutrient dynamics and biota in streams. Influences of discharge (Q), reach-scale channel form, and riparian vegetation on coarse particulate organic matter (CPOM) retention were analyzed in 2 headwater streams in northeastern Oregon. Ginkgo biloba leaves were released in coniferous forest reaches and downstream floodplain meadow reaches during spring high flow and summer baseflow. Transitional reaches were also analyzed during summer baseflow. Paper strips, simulating sedge blade retention, were released in meadow reaches during high flow. Mean transport distances (SP) were calculated as the inverse of the longitudinal loss rate (k) of leaves in transport. The metrics SP, width-specific discharge (QW = Q/stream width), and the mass transfer coefficient (vdep=QW/SP) were used to investigate retention. Values of SP (0.9-97 m) were 2 to 11 times longer during high flow than baseflow. Mean SP in forest reaches (29.3 m) was significantly shorter than in meadow reaches (68.9 m) during high flow but not during baseflow. Standardizing k for the scaling effects of Q by analyzing the relationship between QW and SP, in which the slope equaled the inverse of mean vdep of all Ginkgo releases, indicated times when vdep was higher or lower than predicted by Q. Values of SP were driven largely by Q, yet most experiments in which values of vdep exceeded those predicted by QW occurred during high flow. Values of vdep (0.3-32 mm/s) across experiments were generally inversely related to SP but did not differ between forest and meadow reaches during high flow. Unlike meadow reaches, mean vdep in forest reaches was higher during high flow (5.2 mm/s) than baseflow (1.1 mm/s). Values of vdep were positively related to large wood volume and negatively related to the extent of floodplain inundation during high flow. Yet, in the meadow reach that had lower relative channel constraint, paper strips were transported farther onto the floodplain as Q rose, resulting in long-term (∼1.5 mo) retention. Despite downstream increases in Q, there were no differences in mean baseflow SP or vdep among reaches in either stream, indicating some longitudinal compensation in retention. Alternating associations between retention metrics and structural elements of the stream channels between flow periods suggests dynamic reach-scale hydrologic-retention thresholds in response to changes in Q. Analysis of vdep across experiments indicated that channel morphology, stream wood, and riparian vegetation are major controls on CPOM retention.