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5.7 Delivery/ConnectivityIntroduction/Background
Delivery or connectivity can be considered in context with processes such as landslides and debris flows. NetMap contains a couple of tools for evaluating the delivery of sediment and large wood to streams from landslides and debris flows. Delivery of sediment is also considered for hillslope surface erosion (Disturbed WEPP), WEPP road surface erosion and GRAIP-Lite road surface erosion.
Debris flows are a significant component of sediment budgets in many mountain landscapes (Dietrich and Dunne 1978; Swanson et al. 1982; Wohl and Pearthee 1991) and they can cause erosion to be highly episodic (Benda and Dunne 1997a; Kirchner et al. 2001). In certain landscapes, low-order channels prone to debris flows comprise up to 80% of a channel network, and hence network topology can influence the spatial diversity of channel and valley morphologies (Benda and Dunne 1997b). In depositional areas, debris flows construct levees (Canon and Bonny 1902); build fans at tributary junctions (Dietrich and Dunne 1978); create boulder deposits along fan margins (Benda 1990; Wohl and Pearthree 1991); form ponds at fan constrictions (Everest and Meehan 1981); create wide valley floors (Grant and Swanson 1995); force channel meanders (Benda 1990); and spates of debris flows can lead to widespread channel aggradation and formation of terraces (Wohl and Pearthee 1991; Robert and Church 1986; Miller and Benda 2000). In addition, debris flows can incorporate logs and whole trees that have accumulated in low-order channels over decades to centuries and they deposit them on fans, valley floors, and in channels at low-order confluences (Swanson and Lienkaemper 1978; Hogan et al. 1998; May 1998).
Road construction and timber harvest have increased the occurrence of debris flows in some landscapes by lowering hillslope stability (Burroughs and Thomas 1977). In some cases, debris flows have increased their travel distances due to the absence of large trees along flow paths (Ketcheson and Froelich 1978; May 1998). Increased occurrence of debris flows due to timber harvest and logging roads has raised concerns about impacts to fishes and other aquatic life. Negative effects of debris flows may include immediate burial of existing habitat and direct mortality of aquatic biota; increased fine sediment in gravels onsite and downstream that suffocates fish eggs in gravel (Everest et al. 1987; Scrivener and Brownlee 1989); increased bedload transport and lateral channel movement due to heightened sediment supply that scours fish eggs; and loss of pools that reduces rearing habitat (Frissel and Nawa, 1992; Hogan et al. 1998).
Debris flows are also a “disturbance” when viewed by ecologists (Swanson et al. 1988) and the pivotal role of certain types of disturbances in maintaining productivity and diversity in aquatic ecosystems is gaining increasing recognition (Resh et al. 1988, Reeves et al. 1995). Beneficial effects of debris flows on aquatic systems include formation of ponds that become occupied by fish and beaver (Everest and Meehan 1981); release of nutrients due to buried organics in anaerobic environments (Sedell and Dahm 1984); deposition of woody debris that creates sediment wedges and forms pools (Hogan et al. 1998); deposition of boulders that trap sediments and create complex habitats (Reeves et al. 1995); formation of wider valley floors that contain larger floodplains (Grant and Swanson 1995); and increased biological productivity (Roghair et al. 2002). Spates of debris flows may also contribute to watershed-scale habitat diversity, including in riparian forests (Nierenberg and Hibbs 2000; Nakamura and Swanson 2002).
To gain a longer-term perspective on episodic erosion in humid mountain landscapes, the occurrence of debris flows and sediment routing at the scale of a sixth- order network over centuries was investigated using computer models (Benda and Dunne 1997; U. S. F. S 2002). The models predict that large volumes of sediment are concentrated in parts of the channel network, particularly near tributary junctions (i.e., low-order to high-order confluences), during infrequent periods of intense hillslope erosion following forest fires and large storms. The concentration of sediment at low-order confluences was referred to as ‘stationary sediment waves’ to extend the definition of fans to include their temporally variable influences on channel morphology and to distinguish them from waves of sediment that migrate through a network. A later theoretical analysis of the long-term mass balance of in-stream wood indicated the potential importance of debris flows as a wood recruitment agent in larger channels (Benda and Sias in press), a prediction that was preceded and motivated by field studies indicating the same conclusion (Swanson and Lienkaemper 1978; Hogan et al. 1998).
Despite the history of field work and modeling, the effects of debris flows on channel and valley morphology, particularly as it pertains to riverine ecology, remain an outstanding question. For example, none of the existing models for predicting the occurrence of shallow landslides (Montgomery and Dietrich 1994) and transport distance of debris flows (Benda and Cundy 1990, Fannin and Wise 2001) consider or predict the morphological or ecological consequences in rivers. This reduces the ability of geomorphology to effectively participate in interdisciplinary collaborations that are aimed at understanding natural environments and human alterations of them (Benda et al. in press). In addition, although the potential for tributary confluences to play a key role in riverine ecology has been acknowledged, little supportive empirical information exists (Fisher 1997).
Debris Flows as Sources of Large Wood to Stream Channels (and see NetMap's tool for upslope sources of wood to streams)
Trees are recognized as an important element influencing debris-flow behavior. A variety of evidence suggests that trees can reduce the potential for debris-flow-triggering landslides (Schmidt, Roering et al. 2001; Roering, Schmidt et al. 2003) and that debris flows traversing unforested areas, or areas with young forests, tend to travel further than similar debris flows through old forests with large trees (Robison, Mills et al. 1999; May 2002; Ishikawa, Kawakami et al. 2003; Miller and Burnett 2008). Theoretical arguments also suggest that incorporation of wood (from standing or down trees) into debris-flow material will reduce runout length (Lancaster, Hayes et al. 2003).
Trees also have a profound influence on the consequences that debris flows pose to the channels in which they deposit material. Wood carried to channels by debris flows can persist for decades, potentially providing long-lasting sources of jam-forming key pieces and roughness elements that can add to channel complexity (Benda 1990; Benda, Veldhuisen et al. 2003; Bigelow, Benda et al. 2007). The consequences of debris-flow deposition vary over a channel network, depending in part on characteristics of the receiving valley and channel (Benda 1990; May and Gresswell 2004). Debris-flows can be a primary source of in-channel wood in portions of the channel network bordered by steep, landslide-prone hillslopes (Benda, Bigelow et al. 2002; May and Gresswell 2003; Reeves, Burnett et al. 2003). When the effects of debris flows are considered over entire channel networks and over time, they are seen to be an important component in the temporal and spatial variability (Benda, Miller et al. 1998; Benda, Andras et al. 2004) to which Pacific Northwest ecosystems have evolved (Reeves, Benda et al. 1995). Past management practices, including stream cleaning, splash damming, road construction, and harvest in riparian and debris-flow-prone zones, has resulted in a reduction of in-channel wood (Czarnomski, Dreher et al. 2008) and shifts in the frequency, locations, wood content, run-out extent, and effects of debris flows (Lancaster, Hayes et al. 2001; Lancaster, Hayes et al. 2003; Montgomery, Massong et al. 2003). I’m citing the concepts and references I’m most familiar with here; if there are others to consider, please post a comment to tell us about them.
Current management strategies seek to re-establish and maintain natural watershed processes (Rieman, Dunham et al. 2006; Bisson, Dunham et al. 2009; Waples, Beechie et al. 2009), including actions to ensure that when debris flows do occur, they carry wood and include a size distribution of wood pieces reflective of natural conditions (see, for example, chapter 4 in the Northwest Oregon State Forest Management Plan). It is useful, therefore, to identify source areas for debris-flow-carried wood.
Debris flows do incorporate entire trees: those standing at the initiating landslide can be carried away, roots and all. As debris flows travel downslope through steep, low-order channels, they also pick up wood that has accumulated over time (May and Gresswell 2003). Wood can collect in these channels for centuries, and debris flows can scour material over significant channel lengths (hundreds to thousands of meters). Hence, a large proportion of debris-flow-carried wood comes from trees that fall into these low-order channels and from trees and down wood carried into these channels by small, channel-adjacent landslides.
Debris flows are typically triggered in unchanneled and steep areas in watersheds and they traverse headwater streams and deposit sediment and woody debris (including who trees) into fish bearing streams. Debris flows occur in old growth forests during intense rainstorms (Figures 1 and 2), following fires (Figure 3) and following timber harvest and road construction (Figure 4). In all of these environments, debris flows can represent a risk to aquatic resources. Negative effects of debris flows may include immediate burial of existing habitat and direct mortality of aquatic biota; increased fine sediment in gravels onsite and downstream that suffocates fish eggs in gravel (Everest et al. 1987; Scrivener and Brownlee 1989); increased bedload transport and lateral channel movement due to heightened sediment supply that scours fish eggs; and loss of pools that reduces rearing habitat (Frissel and Nawa, 1992; Hogan et al. 1998).
Landslides and debris flows may also have a positive effect on aquatic ecosystems. Beneficial effects of debris flows on aquatic systems include formation of ponds that become occupied by fish and beaver (Everest and Meehan 1981); release of nutrients due to buried organics in anaerobic environments (Sedell and Dahm 1984); deposition of woody debris that creates sediment wedges and forms pools (Hogan et al. 1998; Reeves et al. 2006; Bigelow et al. 2007); deposition of boulders that trap sediments and create complex habitats (Reeves et al. 1995); formation of wider valley floors that contain larger floodplains (Grant and Swanson 1995); and increased biological productivity (Roghair et al. 2002). Spates of debris flows may also contribute to watershed-scale habitat diversity, including in riparian forests (Nierenberg and Hibbs 2000; Nakamura and Swanson 2002; Bigelow et al. 2007).
 Figure 1. A 1939 aerial photograph of an old growth forest in the Olympic Peninsula, Washington, shows how a very large storm (1934) triggered numerous debris flows at the heads of first order streams. Most of the debris flows traversed through headwater (first and second order) streams and deposits sediment and wood in higher order fish bearing streams (from Benda et al. 1998). This type of watershed has high connectivity.
 Figure 2. An example from an old growth forest in the Oregon Coast Range (right panel) shows accumulations of large logs that was deposited by a debris flow at the mouth of a second order streams; see also Bigelow et al. 2007.
 Figure 3. Stand replacing fires in the Tillamook basin, Oregon Coast Range, in conjunction with large rainstorms triggered numerous shallow landslides and debris flows (from Benda and Dunne 1997a). Wildfires can increase the connectivity between hillslopes and streams.
 Figure 4. An extreme example of poor road building, widespread clearcutting and a large storm yielding numerous debris flows that deposited sediment and organic material into fish bearing streams (in northwest Washington). This highly dissected landscape has great potential for very high connectivity, particularly following the destabilizing effects of road construction and timber harvest.
NetMap contains a tool for predicting debris flows and their potential ecological impacts on streams and rivers.
In this model, there are two types of source areas for large wood to fish bearing streams and rivers:
1. hillslope areas that trigger debris flows that travel to fish-bearing streams, and
2. steep headwater channels that are traversed by debris flows prior to depositing in lower gradient, fish bearing channels.
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