The Geomorphic Significance of Log Steps in Forest Streams¹

Abstract

A functional account of log steps in forest streams is provided by field surveys of 163 kilometers of streams in the central Oregon Coast Range. Natural treefall, rather than silvicultural activities, accounts for the majority of log steps. During low-flow conditions, dissipation of potential stream energy by log steps amounts to 6 percent, approximately equal to that by falls. There are no statistically significant differences regarding spatial distribution of log steps between study basins with contrasting silvicultural and natural stream inputs of large woody debris. However, significant spatial differences are revealed between streams of various orders, a finding that points to channel flushing capacity and stream-adjacent topography as dominant controls on log step development. Application of thermodynamic principles to stream systems demonstrates that neither falls nor log steps cause a statistically significant difference in equilibrium conditions of stream networks. The volume of sediment stored behind log steps in third-, fourth-, and fifth-order streams is 123 percent of the mean annual sediment discharge (suspended load and bed-lood). Depriving some streams of log steps by stream clean-out or repeated harvest of stream-adjacent trees may initiate an episode of progressive erosion by not dissipating stream energy in excess of that needed to transport imposed sediment supplies. Addition of log steps to streams with energy already insufficient to balance sediment inputs and outputs may only serve to accentuate progressive deposition. Functions of instream large woody debris not incorporated as log steps must also be addressed in forest management decisions.

The permanence of debris dams related to river channel processes

“…Heede, 1980Heede, , 1981, these organic accumulations have not previously been investigated in detail in Britain and there has been comparatively little attention paid to the short-term changes, spatial extent, stability and permanence of the dams. These are aspects which have not previously been studied elsewhere, although there have been general summaries of dams per unit length of channel (Marston, 1982) and detailed maps (Keller & Tally, 1979;Megahan, 1982) and long profiles (Mosley, 1981) have been produced for small reaches, and for reaches in which tree roots cross the entire channel (Zimmerman et al, 1967). No studies have hitherto investigated the permanence of debris accumulations throughout a channel network.…”

“…Nevertheless, logs and trees can also focus the accumulation of gravel bars along larger channels (Nanson, 1981) and during extreme floods temporary dams and log jams can accumulate in very large rivers (Dobbie & Wolf, 1953;Hickin & Nanson, 1975). Debris dams have been shown to exercise a significant effect upon channel processes by reducing stream velocity and discharge (Heede, 1981) by affecting flow routing and by increasing sediment storage with, for example, up to 123% of the mean annual sediment discharge stored behind log steps in the Oregon Coast Range (Marston, 1982), and 15 times the average annual sediment yield behind dams on small streams on the Idaho batholith (Megahan, 1982). Thus sediment storage resulting from debris dams provides a buffer in the sediment routing system.…”

“…Such research has been undertaken particularly in North America and has shown how the vegetation dams can affect the detailed morphology of particular reaches (Keller & Tally, 1979) and how the dams can exercise a very important influence throughout headwater channels with dams occurring every 1.5-8 m of channel in the western USA (Heede, 1981). A considerable proportion of the vertical fall of channels can occur at the sites of debris dams, accounting for 60% of the total drop in some streams such as Little Lost Man Creek in the coastal Redwood environment of California (Keller & Tally, 1979); for a cumulative height of 392 m along 163 km of forest streams in central Oregon (Marston, 1982); and for influencing the precise location of 30-80% of the fall of small streams in western Oregon (Keller & Swanson, 1979). Debris dams are particularly significant in headwater streams so that along New England streams (Likens & Bilby, 1982) the size of organic debris dams depends primarily upon the size of the tree and of the logs available.…”

Debris dams and the relief of headwater streams

“…These impoundments typically fill with sediment to form steps and force nominally bedrock channels to be alluvial (Montgomery et al, 1996(Montgomery et al, , 2003bMassong and Montgomery, 2000; figure 2). Wood thus affects amounts and patterns of sediment storage in drainage networks (Swanson and Lienkaemper, 1978;Marston, 1982;Nakamura and Swanson, 1993;Lancaster et al, 2001;May and Gresswell, 2003) and also affects smaller features such as the locations and morphologies of pools (Buffington et al, 2002). Montgomery et al (2003a) surmised that the relative immobility of large wood, its impounding of sediment, and the concomitant shielding of the underlying bedrock decrease the effective bedrock erodibility and, hence, steepen the longitudinal channel profile.…”

Effects of Spatially Distributed Stream Power on Check Dam Function in Small Upland Watersheds: A Case Study of the Upper Laja Watershed, Guanajuato, Mexico

“…Self-organizing step-pool sequences occur in a variety of fluvial settings, ranging from perennial streams in temperate climates to ephemeral streams in semi-arid and hyperarid regions (Wohl and Grodek, 1994;Chin, 1999;Chin and Phillips, 2007). Check dams, like naturally-occurring log steps in forested alpine streams, serve to dissipate the potential energy within concentrated flow that would otherwise be used to transport sediment downstream (Marston, 1982;Wilcox et al, 2011). As a consequence, local streamflow velocities are reduced and water surface slopes are decreased on the upstream side of a check dam (Shieh et al, 2007).…”