Summary of in-channel wood in the Colorado Front Range
(Information compiled by Dan Cadol)
Wood is a major control on stream channel morphology (e.g., Brooks et al., 2003; Kail, 2003; May & Gresswell, 2003b) and stream ecosystem habitat diversity (e.g., Bisson et al., 1987; Maser and Sedell, 1994 ). Wood is recruited into streams through individual tree mortality and larger wood inputs such as debris flows and avalanches (May and Gresswell, 2003a). If stream flow is locally unable to transport a piece of wood, the wood may persists for many decades, altering flow patterns and trapping sediment, until decay or abrasion reduces its size or strength enough to enable breakage or fluvial transport. If a wood piece is transported by the flow, it will commonly travel downstream until it becomes caught on a larger log, creating a jam. The large logs that initiate jams are referred to as ‘key pieces.’ Stable wood in channels, whether individually or in a jam, corresponds to a higher concentration of pools, cover, and stored fine sediment and organic matter (Raikow et al., 1995; Faustini & Jones, 2003).
Click a link below to find out more about in-channel wood in the Front Range.
Wood Load
Wood load describes the concentration of wood within a stream reach. It is typically measured as either a volume of wood per unit length of channel or a volume of wood per unit area of active channel. Wood volumes measured in the Front Range by Richmond and Fausch (1995) range from 9.1 to 27.1 m3 of wood per 100 m of channel or 92-254 m3 per hectare of channel. Wohl and Jaeger (2008) found loads of 0.1-9.7 m3 of wood per 100 m of channel or 1-218 m3 per hectare of channel in the Front Range. For wood to be included in these measurements, the researchers used minimum size criteria of 1 m length and 10 cm diameter. Higher wood load commonly correlates with higher pool frequency, shorter pool spacing, and increased habitat diversity and complexity at the microhabitat scale.
Beavers
“More than any other animal except humans, beavers geomorphically alter the landscape through their dam building and related activities” (Butler, 1995). Historically, beaver activity was likely a major influence on the amount and distribution of wood in the channel networks of the Colorado Front Range. Beaver were trapped to near- extinction in the 1800’s (Rutherford, 1964), but have reclaimed most of their original habitat since 1900 and are estimated to have recovered to 10% of their pre-trapping density continent- wide (Westbrook, 2005). Beaver ponds provide increased habitat diversity and complexity to the aquatic ecosystem (Naiman et al., 1988; Olson and Hubert, 1994). The dams promote overbank flooding and recharge of local shallow aquifers (Westbrook et al., 2006). The dams and ponds also trap silt and organic material, eventually filling and raising the level of the valley floor (Ives, 1942). The meadows created by filled beaver ponds provide increased habitat diversity in the terrestrial ecosystem, as well.
Disturbance Regime Influences
Disturbances have the potential to dramatically alter wood recruitment rates into channels of the Front Range. Fire, wind, and beetle kill increase tree mortality, potentially increasing short term wood volumes introduced into the channels (Bragg et al., 2000). These events may also cause a decrease in wood loads over longer periods, because as the forest recovers, the local supply of wood to the channel will be dramatically lower than when the forest is mature. Landslides, debris flows, and avalanches have the potential to supply large loads of wood to the channel quite suddenly (May and Gresswell, 2003a). The distribution of wood in a channel network is expected to be closely correlated with the distribution of landslides and debris flows. Material delivered by mass movements commonly forms large dams because the transport capacity of the channel is much lower than the capacity of the mass movement event.
Anthropogenic Wood Removal
Over the past century, human management of in-channel wood has been almost exclusively limited to wood removal (e.g., Maser and Sedell, 1994; Wohl, 2004; Montgomery and Wohl, 2004). Reasons for wood removal that are specific to the Front Range include flood control, infrastructure protection, and increasing the efficiency of the channels to transport timber during tie drives. Timber harvesting also reduces the amount of wood delivered to the channel from the hillslopes. The net effect of various land uses (beaver trapping, timber harvest, tie drives, road construction, flow regulation) in the Front Range has been to reduce wood loads relative to loads present prior to 1800 A.D. (Wohl, 2001). This has in turn likely increased the mobility and decreased the residence time of any remaining wood in the channels (Wohl and Goode, in press).
Dynamiting a log jam, New Brunswick, Canada.
(Photo by W. Crawford)
Infrastructure Protection
The courses of numerous rivers in the Front Range also serve as transportation corridors (e.g., Clear Creek / I-70, Boulder Canyon / CO 119, Big Thompson Canyon / US 34, Poudre Canyon / CO 14). Logs may accumulate on bridge piers and weaken the structures, so transportation departments generally have a policy of removing wood from channels.
Downstream of a recently dynamited splash dam, early 1900s. Wind River, OR.
(Posted by the Oregon Historical Society)
Flood Control
Instream wood increases flow resistance by redirecting flow, forcing flow separation, and creating turbulence and back-waters (Daniels & Rhoads, 2003; Curran & Wohl, 2003) . The overall effect is to reduce the efficiency of the channel in conveying water downstream. During high flows, this decrease in efficiency becomes visible in increased overbank flow (Jeffries et al., 2003) . In reaches where human use of the floodplain makes overbank flow undesirable, managers have frequently worked to make the channel more efficient, thereby increasing the discharge that can be contained within the banks. Wood removal is one strategy employed to increase efficiency. The consequence is a homogeneous channel morphology with few habitat features necessary for complete aquatic ecosystem function.
Wood caught on bridge.
(Miller, 2003)
Big River, CA, 1924.
(Courtesy of the Mendocino Historical Society)
Remnant splash dam (built in 1911) from railroad tie drives in the Bighorn National Forest, WY.
(Photo by A. Nowakowski)
Wood in channel prior to spring flood, South Tongue River, WY.
Oscar A. Granum collection
Tie Drives
The construction of the railroad network in the 19th century required vast quantities of lumber to form railroad ties (Wohl, 2001). Wood was scarce throughout the Great Plains, so railroad companies used timber from mountainous areas such as the Front Range. As the easily accessed forests were quickly depleted, loggers moved farther into the mountains. To solve the difficulty of transporting the timber to the rail head, logging crews would typically work all summer and haul the felled trees to one of the trunk rivers draining the Front Range. A splash dam would then be constructed below the collected timber. During the spring floods, when the reservoir behind the dam had filled and floated the wood, the dam would be dynamited, sending a flood of wood and water down to the plains, where the wood was collected for use. In order to increase the effectiveness of this method, pre-existing wood jams or boulders which might trap wood were sometimes dynamited. The practice of transporting lumber in tie drives effectively homogenized the morphology of the affected channels (Wohl, 2001). Bed and banks were scoured, sometimes to bedrock, and channel irregularities were removed. Click here to link to Land Use page.
High-elevation streams may provide the best understanding of natural wood loads because of their remoteness and poor accessibility for land-use activities. However, wood loads are expected to vary systematically down the channel network, so natural loads for headwater streams may have little bearing on natural wood loads in the main trunk channels that drain to the plains (Wohl and Jaeger, in press). Because of the lack of historical records, the nearly complete deforestation of the Front Range in the late 1800’s, and the continued removal of wood, especially along highway corridors, the natural wood load of the Front Range is unknown. Post-disturbance recovery times of wood loads are expected to be on the order of 200 years (Bragg, et al., 2000).
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