Wood dynamics in headwater streams of the Colorado Rocky Mountains

Abstract

[1] An 11-year monitoring study begun in 1996 evaluates wood mobility in five Colorado Rocky Mountain streams. Each channel reach is 40-70 m in length. Initial surveys included channel dimensions and arrangement of each piece of wood. Annual resurveys focused on wood arrangement and persistence. Average diameter of 15 cm and length of 3.2 m varied little among sites. Average yearly mobility ranged from 16% to 23%. Average residence time was 3.4 years. Wood load correlates positively with valley width and channel gradient, and negatively with relative substrate submergence and mean annual peak discharge. Survival analysis revealed that individual piece residence time was controlled by a nondimensional piece length and peak unit stream power during the year of removal. Residence time increases as piece length relative to channel width decreases, which could be explained by a greater integration of the flow field for longer pieces. Mobility of individual pieces of wood is controlled primarily by piece characteristics (length, diameter, type). Likelihood of mobility is smallest for buried pieces. The brevity of residence time relative to studies from other regions likely results from relatively low wood loads in these channels after timber harvesting cleared both instream and riparian wood supply more than a century ago. Although individual pieces of wood are exchanged, wood load and the location of individual logjams remain relatively constant.

Factors influencing wood mobilization in streams

“…Burial and unburial can remove or return a substantial amount of wood to streams [Latterell and Naiman, 2007]. Although few studies have considered the effects of burial on mobilization, when burial is included in analyses, it is of primary importance [Berg et al, 1998;Wohl and Goode, 2008]. Our study demonstrated the importance of burial to mobilization using a quantitative analysis and a large number (865) of wood pieces.…”

“…At the 10 m scale, the length ratio may relate to the probability that a piece becomes braced before it can escape the 10 m section. Most previous studies have likewise found a negative relationship between length ratios and mobilization [Bilby, 1984;Lienkaemper and Swanson, 1987;Berg et al, 1998;Jacobsen et al, 1999;Gurnell et al, 2002;Hassan et al, 2005;Warren andKraft, 2008, but see Wohl andGoode, 2008]. Finally, we used the effective channel width (i.e., the unobstructed wetted width available to transport wood) to determine our length ratio rather than bankfull width or wetted width.…”

“…Thus, the results from this study are applicable to at least a similar range of conditions in other streams. Four of the final seven predictor variables take discharge into account (i.e., effective depth, length ratio, downstream force ratio, draft ratio), and the remainder have been identified as important in other settings [Berg et al, 1998;Braudrick and Grant, 2000;Wohl and Goode, 2008].…”

“…[13] Pieces of wood may be partially buried in the stream bed; some studies suggest that burial is the most important determinant of mobilization [Berg et al, 1998;Wohl and Goode, 2008]. A partially buried piece requires a greater force to mobilize it compared to an exposed piece [Brooks et al, 2006].…”

“…[5] Wood mobilization in natural streams has been investigated in a number of field studies [Bilby, 1984;Lienkaemper and Swanson, 1987;Berg et al, 1998;Jacobsen et al, 1999;Warren and Kraft, 2008;Wohl and Goode, 2008]. The most frequently cited predictor of mobilization that has emerged from these studies is the ratio of the length of a wood piece to the bankfull width of the stream channel [Gurnell et al, 2002;Hassan et al, 2005].…”