DEBRIS DAM DYNAMICS AND COARSE PARTICULATE ORGANIC-MATTER RETENTION IN AN APPALACHIAN MOUNTAIN STREAM

Debris dam structure and retention of coarse particulate organic matte r were examined during a 17-mo period in Powdermill Run, a 3rd-order A ppalachian Mountain stream. Through the use of detailed feature maps, changes in debris dam morphology were recorded, including the complete ''life-cycle'' (i.e., initial formation to destruction) of a dam. Str eam sections in which dams were naturally destroyed became markedly le ss retentive. Leaves were used as tracers in retention experiments tha t varied in duration from 3 h to 4 wk. Results implied that migration over time occurred by a simple mechanism of leaves falling off rocks a nd settling into debris dams. A series of releases over 12 d showed in creasing retentiveness as discharge decreased: Seasonal differences in retention potential were evaluated using 3-h releases conducted durin g winter, summer, and autumn Summer was the most retentive season due to base-flow conditions. Debris dams were most retentive in autumn,les s so in winter, and least retentive in summer. Cobbles showed the oppo site pattern Leaf retention ranged from 1.8 to 23.2% retained/m (-k: 0 .02 to 0.26), depending on season A significant negative relationship was found between mean depth and % retained/m, but the relationship of % retained/m to discharge was not significant. A consideration of sea son is necessary when comparing retentive abilities between streams.