In forested channel networks wood can be the dominant control on grade and shear stress partitioning. In these systems a loss of wood triggers channel incision that results in a lowering of alluvial groundwater
tables and loss of water storage within a watershed. It also speeds up the routing of water out of channel network. Alluvial channels consist of two distinct channels conveying water: i) a surficial channel of open channel flow and ii) a subsurface hyporheic groundwater “channel” of flow moving through a porous medium of alluvial sediment. The velocity of water moving through these two pathways varies by up to five orders of magnitude. Thus, the subsurface flow volume can have a significant role in water retention within basin as well as supplementing baseflows. Channel incision leads to a substantial reduction or even complete loss of subsurface water capacity by lowering water tables and evacuating alluvial sediment.
Shields et al (2009) report that 60-90% of sediments leaving many watersheds are due to channel incision. There are several major causal mechanisms triggering channel incision (e.g., Schumm et al. 1984). Dams cut off sediment supply which will drive downstream incision without a major reduction in peak flows (e.g., Galay 1983, Williams and Wolman 1984, Ligon et al 1995, James 1997, Kondolf 1997). Another mechanism of channel incision is changes in flow regimes that increase the magnitude and frequency of peak flows such as urbanization (e.g., Hamer 1972, Booth 1990). Large scale forest clearing can increase channel drainage networks and the frequency of bankfull or bed mobilizing peak flows which can result in channel incision (e.g., Wemple et al. 1996, Prosser and Moufi 1998, Marden et al. 2005, Perry et al. 2016). Channelization and shortening the length of channels also contributes to incision by increasing hydraulic gradients and sediment transport capacity (e.g., Simon 1989, Simon and Rinaldi 2006). In North America, the historic removal of beaver contributed to channel incision through the loss of in-stream wood dams beavers created (e.g., Pollock et al. 2007, 2012, 2014). We believe the most widespread cause of channel incision involves the forest clearing and the loss of in-stream wood either by direct removal or clearing (e.g. Prosser and Soufi 1998, Collins et al. 2002, Brooks et al. 2003, Marden et al. 2005, Stock et al. 2005, Brummer et al. 2006, Montgomery and Abbe 2006, Sear et al 2010, Phelps 2011, Abbe et al. 2016). By definition, this has led to an extensive loss of the natural alluvial and surface water storage that once existed. Figure 1 illustrates a recent example of rapid incision and gully formation after industrial logging. The ecologic benefits of wood placement are well established and are being used around the world (e.g., Abbe and Brooks 2011, USBR and ERDC 2016, Bridges et al. 2018). They can also help to attenuate flood peaks (Anderson 2006, Nisbet 2012, Abbe et al. 2016, Bridges et al. 2018) and reduce organic contaminants (Peter et al. 2019). The focus of this paper is highlight the potential role wood placements can have on raising water tables and substantially increasing the water storage within a
channel network.
For more information contact: Tim Abbe, PhD, PEG, PHG at Natural Systems Design and Coastal Geologic Services — Email: Tim@naturaldes.com | Phone: (206) 834-8697