Transport, retention, and ecological significance of woody debris within alarge ephemeral river

The spatiotemporal patterns and ecological significance of the retention of coarse particulate organic matter and large woody debris have been intensi vely studied in perennial rivers and streams but are virtually unknown in e phemeral systems. We examined the influence of 2 features characteristic of ephemeral systems, downstream hydrologic decay and in-channel tree growth, on the distribution, transport, and retention of woody debris following a flood having a similar to 2.6-y recurrence interval in the ephemeral Kuiseb River in southwestern Africa. A total of 2105 pieces of wood were painted at 8 sites dong the river channel to measure retention patterns. The flood had a peak discharge of 159 m(3)/s at the upper end of the study area, deca ying to <1 m(3)/s by 200 km downstream. Downstream export of wood from mark ing sites totaled 59.5% (n = 1253). Transport distances ranged from 1 to 12 4 km, and 34.8% (n = 436) of the exported wood was recovered. Marked wood r etained within marking sites was significantly longer than exported wood (p < 0.001, t-test). Once in transport, there was little correlation between wood length and distance traveled (r = 0.11, correlation analysis, n = 369) . Length influenced the site of retention; material retained on debris pile s was significantly longer than that stranded on channel sediments (p < 0.0 01, t-test). In-channel growth of Faidherbia trees significantly influenced wood retention; 83.7% of marked wood not moved by the flood was associated with debris piles on Faidherbia trees. Similarly, 65% of the exported wood retained within downstream debris piles was associated with Faidherbia tre es. In contrast to many perennial systems, we observed a general increase i n wood retention downstream, peaking in the river's lower reaches in respon se to hydrologic decay. Debris piles induced sediment deposition and the fo rmation of in-channel islands. Following flood recession, debris piles and their associated sediments provided moist, organic-rich microhabitats, whic h were focal points for decomposition and secondary production, mimicking p atterns reported from the channels of perennial streams and rivers. The eco logical significance of retentive obstacles and associated organic debris i s a feature common to all fluvial ecosystems, irrespective of their hydrolo gic regime.