A conceptual model for the longitudinal distribution of wood in mountain streams

Abstract

Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel-, valley-and watershed-scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non-randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non-randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range.

Climate change, forests, fire, water, and fish: Building resilient landscapes, streams, and managers

“…Dams formed by accumulations of large wood increase channel complexity and facilitate deposition of organic matter, thus providing a food source for numerous invertebrate species and contributing to nutrient cycling and retention Wondzell and Bisson 2003). The influence of wood in affecting stream morphology depends on the size of the stream and the size of the wood pieces (Bilby and Ward 1989;Marcus and others 2002;Wohl and Jaeger 2009). The function of LW in forming fish habitat, especially plunge and dammed pools, is strongly influenced by the location of the stream or reach within a given watershed (Richmond and Fausch 1995).…”

“…The relative importance of chronic LW inputs vs. episodic, disturbance-related inputs varies in time and space (Benda and others 2003b) and is reflected in wood distribution at multiple scales. In one of the few empirical studies to quantify the longitudinal distribution of instream LW, Wohl and Jaeger (2009) surveyed wood pieces in 50 contiguous stream segments, each segment 25 m in length (total surveyed length = 1250 m per stream) along 12 streams in the Colorado Front Range. Their results suggested that local valley and channel geometry, i.e., valley-bottom width, gradient, and sequence of channel changes, exerted a stronger influence on patterns of longitudinal wood distribution than either time since last forest disturbance or progressive downstream trends associated with larger drainage area.…”

“…Conceptual models of LW distribution and dynamics are generally based on a simplified landscape view of stream networks or watersheds, classified into three dominant morphologies: high gradient, small headwaters; intermediate, 3rd and 4th order stream segments; large, low-gradient, meandering streams and rivers (Marcus and others 2002;Swanson 2003;Wohl and Jaeger 2009 and see text box on large wood dynamics). As noted above, vegetation, physical constraints, and natural hydrologic, sediment, and disturbance regimes differ markedly in these portions of river and riparian landscapes and strongly influence LW distribution and dynamics.…”

Watershed controls on the export of large wood from stream corridors

“…Significant recruitment from upstream hillslopes coupled with episodic floods that cause bank erosion and transport capacity downstream could synergistically increase upstream rate of export while decreasing the export rate with downstream changes to the floodplain dynamics (Marcus et al 2002;Reeves et al 2003;Seo and Nakamura, 2009;Wohl and Jaeger, 2009). …”

“…A watershed approach to large wood dynamics is valuable for understanding the multi-scale drivers of large wood and connectivity among watersheds (Wohl and Jaeger, 2009). However, quantifying the large-scale patterns of large wood export is hampered by the highly variable and discontinuous recruitment and transport of large wood during high flow events (Moulin and Fremier, Seo and Nakamura Revised Manuscript -October 4, 2009 Page 5 of 39 Piégay, 2004;Comiti et al, 2006;Young et al, 2006).…”