DRIVERS OF COARSE PARTICULATE ORGANIC MATTER TRANSPORT AND STORAGE IN HEADWATER STREAMS

Coarse particulate organic matter (CPOM), defined as organic matter approximately 1-100 mm in diameter, provides a significant energy source to aquatic ecosystems and is an important component of watershed carbon cycling and export. CPOM transport redistributes organic matter and exports carbon from areas of high inputs, whereas CPOM stored in rivers becomes available to aquatic organisms as a major food source, thereby boosting energy levels and ecosystem health. Previous studies have identified in-stream retentive features and stream flow conditions as key factors influencing CPOM transport and storage. However, our understanding of CPOM transport and storage under diverse geomorphic settings and hydrologic conditions remains limited. We assess the impacts of geomorphic setting, discharge variation, and reach-scale density of retentive features (e.g., downed wood) on CPOM transport rates in two headwater streams within the Boulder Creek Watershed, CO, USA. Gordon Gulch drains an area of 3.6 km² within the montane elevational zone, whereas Como Creek drains an area of 4.9 km² within the subalpine and alpine elevational zones. Additionally, we quantify CPOM storage at sites within Gordon Gulch and assess the relative influence of slope, reach scale retentive feature density, single versus multi-threaded channel planform, and perennial versus intermittent hydrologic regimes on CPOM storage. We also identify the primary trapping mechanisms of CPOM accumulations. Our results show a link between CPOM transport rates and flow conditions, with higher discharge associated with greater CPOM transport, and higher transport during the rising limb of the snowmelt hydrograph than during the falling limb. CPOM transport is significantly greater at Como Creek than at Gordon Gulch, suggesting a potential link between drainage area and flow regime and CPOM transport. Additionally, we find that small wood and twigs are highly important in retaining and storing CPOM in Gordon Gulch, indicating that wood of all sizes is effective at retaining CPOM even in small channels. Our results contribute to the understanding of CPOM within headwater streams and have important implications for watershed management related to carbon cycling and ecosystem functioning.