RELATIONSHIPS BETWEEN CHANNEL MORPHOLOGY AND LARGE WOOD: A LOOK AT NATURAL AND ANCHORED WOOD IN THE CARMEL RIVER, CA

Large woody debris (LWD) is cited as a key component of riverine ecosystems and as a factor in shaping channel-floodplain systems. Despite the natural benefits of LWD, it is commonly removed from rivers within urbanizing watersheds for safety and perceived erosion risk. Conversely, LWD has been widely and variously used in river enhancement projects to foster ecosystem health, induce step-pool formation, resist bank erosion, and to induce bed scour and sediment transport. Watershed management policy would be better informed by studies of the natural distribution and function of LWD. We studied the physical function of LWD in the lower 23 km of the Carmel River, CA.

In our study, every piece of LWD (> 15 cm diam. and > 1.5 m long) lying in or near the bankfull channel was inventoried between the mouth of the Carmel River and a point 23 km upstream. For each occurrence, UTM coordinates and 19 other variables were recorded and entered into GIS. Variables included size, species, orientation to bank, and geomorphic effect. Of approximately 470 instances of single or multiple LWD occurrences we recorded, 57% had no clear geomorphic influence, 29% were associated with bed scour, 13% appeared to protect the bank from erosion, 3% were apparently inducing bank scour, and some LWD had multiple functions. The majority of the LWD providing bank protection was oriented parallel to the bank, or pointed downstream.

Two large-scale LWD enhancement projects with grossly differing geometries were implemented and monitored. In 1999, 36 Eucalyptus logs were installed on the outside of a bend, in clusters of 4 or 5 logs. The structures pointed downstream, approximately 30 to 45 degrees from the bank. This first LWD project was gradually buried by the sediment it trapped, and now presents a high-quality floodplain wetland setting where there was originally an over-widened bankfull channel. In 2002 five large redwood logs were anchored with rootballs pointing upstream at approximately 20 to 30 degrees from the bank. The logs were placed on alternating sides of a straight reach to induce local pools and reduce bankfull width-to-depth for higher sediment transport. This second LWD project has remained stable in greater than bankfull flows and has achieved its goals very well.