In order to provide scientific information on riparian forest management and stream habitat conservation at the watershed scale, characteristics and function of in-stream large woody debris (LWD) and channel morphology in 35 first through fifth order streams of southern interior of British Columbia were surveyed. Study streams through the channel networks of the watersheds were classified into four categories based on stream order and bankfull width: Stream size I: bankfull width was less than 3 m; Stream size II: 3-5 m; Stream size III: 5-7 m; Stream size IV: larger than 7 m. LWD and channel features were compared between the four stream categories. We found that the average diameter, length, volume and biomass of individual LWD pieces increased as a function of increasing bankfull width. However, LWD density (piece per 100 m2 of the stream area) decreased with an increase in bankfull width. LWD volume ranged from 0.78 to 1.58 m3 per 100 m2 of stream area, with intermediate sized streams (II and III) having the largest value and large sized streams (IV) having the lowest values. Results showed that LWD biomass averaged 383 kg/100m2 (range 265-651 kg/100m2) in stream size I, and increased to 491 kg/100m2 (range 81-1254 kg/100m2) in stream size II, and slightly decreased to 465 kg/100m2 (range 247-938 kg/100m2) in stream size III and further decreased to 250 kg/100m2 (range 88-533 kg/100m2) in stream size IV. The contribution of LWD pieces to pool formation was 75 and 85% in stream sizes II and III, respectively, which was significantly higher than those in stream size I (50%) and size IV (25%). The ratio of pool to riffle indicated that pools were shorter than riffles in all steam size categories. Approximately 21 and 25% of LWD pieces were associated with storing sediment, and 20 and 29% of pieces were involved in channel bank stability in all study streams. The large majority of LWD pieces in the smallest sized streams was orientated perpendicular to streamflow and was located above the bankfull height of the channel cross-section. Conversely, most of LWD pieces in intermediate sized streams were orientated parallel to the direction of flow and were situated below the bankfull height of the channel. The variability in the morphological and ecological role of LWD in different sized streams of the watersheds revealed the LWD can be as an indicator of conservation and management of riparian forests at the watershed level. These results also highlight the need to recognize spatial variation of in-stream LWD loading and function through channel network when maintaining suitable LWD pieces and making riparian management decisions at watershed scales.
Wei, Adam. 2005. In-stream LWD as a sustainability indicator at watershed scales for headwater streams of the BC interior. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report