Detritus from riparian vegetation (allochthonous CPOM) is an important resource in stream systems, providing habitat and food for aquatic fauna. Increasing a stream’s ability to trap CPOM (i.e. retention capacity) is, therefore, a common ecological target for river restoration projects. Improving retention capacity is one of several ecological benefits of re-snagging, and over the past 50 years river managers have taken strides to replace LWD and native vegetation removed from channels since European settlement.
However, re-snagging can be expensive, time-consuming and labour-intensive, and it is unclear whether LWD traps more CPOM than other in-stream structures such as macrophytes, debris dams, backwaters and boulders. My study examines the effectiveness of these alternative structures at trapping CPOM, and whether they might be used to improve stream retentive capacity. I hypothesised that structures other than LWD would contribute significantly to overall CPOM retention in headwater reaches in central Victoria.
I compared CPOM retention (CPOM density (m2/m2), total dry mass) between seven types of retention structures in four upland creeks in the Strathbogie Ranges. Sites were large enough to contain multiple retention structures, but small enough for all retention structures to be sampled (100 - 150 m2; n = 22). Potentially confounding hydrological and geomorphological variables (e.g. flow velocity and bed substrate) were similar at all sites (linear regression; P > 0.05) and so were unlikely to affect the results.
Preliminary results show that debris dams accounted for 57% of the total dry mass of CPOM retained (sites pooled), more than three times the amount retained by LWD (17% of total CPOM dry mass). Practices that focus on debris dam initiation and construction may, therefore, complement current re-snagging efforts and provide a cost-effective alternative to river managers.