Influence of large woody debris on channel morphology in native forest and pine plantation streams in the Nelson region, New Zealand

Abstract

The influence of large woody debris (LWD) on channel morphology was assessed in five pine plantation and five native forest streams in the Nelson region of New Zealand. LWD volumes averaged 127 m3 ha–1 in pine plantation streams and 94 m3 ha–1 in native forest streams. Most of the LWD in pine and native streams had no influence on channel morphology (78 and 54%, respectively). Those LWD pieces influencing channel morphology contributed mainly to sediment storage in both pine and native streams as well as flow deflection and debris collection in the native streams. Wood aligned either perpendicular or obliquely to stream flow and positioned on or partly buried in the stream bed had the greatest influence on channel morphology. There were twice as many pools in native streams as in pine streams, and scour processes formed the majority of pools. LWD influenced c. 50% of pool formation in both pine and native streams and increased pool variety. LWD volumes in these streams are low compared with Pacific north‐west streams of the United States, similar to streams in the subalpine forest of Colorado, and higher than volumes in the more modified streams of Europe. Similarities in pool forming processes, pool type, and spatial arrangement of wood influencing channel morphology are apparent between the streams in this study and streams of the Pacific north‐west and Colorado.