Chapter 6 Monitoring Floodplain Restoration

Chapter 6 Monitoring Floodplain Restoration George R. Pess, Sarah A. Morley Northwest Fisheries Science Center, National Marine Fisheries Service 2725 Montlake Boulevard East, Seattle, Washington 98112, USA [email protected] Julie L. Hall, Raymond K. Timm University of Washington, School of Fishery and Aquatic Sciences Box 355020, Seattle, Washington 98195, USA [email protected] Introduction River corridors are naturally dynamic and ecologically complex components of a watershed and often contain a disproportionately high amount of the total regional biodiversity (Naiman et al. 1993; Ward et al. 2001). Unaltered river corridors have heterogeneous landscape features, dominated by dynamic conditions, and exhib- it scale-dependent biophysical patterns and processes (Ward et al. 2001). A prominent feature within river corridors is the floodplain (Figure 1). Geomorphologists traditionally define a floodplain as a flat, depositional feature of the river valley adjoining the river channel, formed under the present climate and hydrologic regime and during times of high discharge (Leopold et al. 1964; Dunne and Leopold 1978; Leopold 1994). Hydrologists and engineers view the floodplain either as land subject to periodic flooding or the area flooded by the 100-year flood event (Dunne and Leopold 1978). Ecologists have defined the floodplain and accompanying habitats as areas that are periodically inundated by the lateral overflow of river or lakes, or direct precipitation or groundwater; the resulting physiochemical environment causes the biota to respond by morphological, anatomical, physiological, phonological, or ethnological adaptations, and produce community structures (Junk et al. 1989). Some of the most common features and terms associated with the definition of floodplains regardless of dis- cipline include main channels; oxbow lakes; point bars; meander bends; meander scrolls; floodplain channels, such as sloughs, beaver ponds, surface and groundwater-fed tributaries, natural levees, or raised berms above the floodplain; wetland areas created by finer sediment overbank deposits; coarser sand deposits called sand splays; accumulations of wood deposits, such as logjams; mid-channel islands created by obstructions, such as wood deposits; and unique vegetation patterns determined by flows, obstructions within the floodplain, and small changes in elevation (Figure 2; Table 1; Leopold et al. 1964; Dunne and Leopold 1978; Wohl 2000). These floodplain-associated features are unique and are needed to help maintain river dynamics, watershed processes, and community structure and function (Ward et al. 2001). The objective of our chapter is to describe how to monitor the effectiveness of floodplain-associated projects that attempt to reconnect isolated habitats. First, we identify and briefly review common effects of anthropogenic disturbance to floodplains and restoration techniques used to reconnect floodplain habitats isolated by anthropogenic disturbance. Next, we discuss how developing a monitoring plan based upon clear restoration goals and objectives can be used to help guide the monitoring of individual or multiple floodplain recon- nection projects. Then, we discuss the selection of physical and biological study parameters and sampling pro- tocols that can be applied to different types of restoration approaches. Finally, we provide recommendations for applying these monitoring techniques to temperate rivers. Pess et al. 128 (a) (b) Figure 1. The Taiya River, southeast Alaska, an example of an anthropomorphically undisturbed floodplain: (a) oblique view, and (b) schematics of how this anastomosing channel appears in cross section. (Photograph and schematic courtesy of Tim Abbe.) Common Effects of Anthropogenic Disturbances to River–Floodplain Systems Anthropogenic disturbances such as dams, levees, and the development of floodplains for agricultural, industri- al, and residential use have disrupted the natural connection of many large rivers from their floodplains, reduc- ing the interaction between lotic, lentic, riparian, and groundwater systems (Petts 1990; Ward and Stanford 1995). Hydrologic flow regulation by dams disrupts the downstream river system’s natural disturbance regime (Ward and Stanford 1995). Such disruptions reduce and alter the frequency, extent, and duration of floodplain inundation, and truncate the input of sediments, nutrients, and wood into and out of the floodplain (Junk et al. 1989; Leopold 1994; Sparks 1995; Ward and Stanford 1995; Collins et al. 2002). This can disrupt the natural connection between rivers and their floodplain habitats (Bednarek 2001). For example, flood-control dams have changed the McKenzie River in Oregon from a multichannel river–floodplain system with midchannel bars and forested islands to a single-threaded channel (Ligon et al. 1995). Ligon et al. (1995) attribute the loss of these features to the reduction of peak flows that historically accessed the floodplain and cut new channels, recruiting the necessary sediment from local sources such as streambanks and terraces, which then deposited downstream and diverted flow. There also is evidence that such midchannel bars and islands were formed primarily from the recruitment of wood from local and upstream sources (Abbe and Montgomery 1996; Abbe 2000). Similar changes have occurred throughout watersheds in North America (Lowery 1968; Harvey et al. 1988). Another common form of anthropogenic disturbance to the connection between main channels and their floodplains is the construction of levees. Levees are artificial embankments, normally higher than the river terrace banks, built close to the edges of riverbanks (Bolton and Shellberg 2001). Levees increase the flow capac- ity of a channel and contain floodwaters within an area narrower than the natural floodplain (Hey 1994). Rea- sons for constructing levees and modifying a river–floodplain system include flood control, increasing the speed of water conveyance during high flows, drainage of adjacent lands, navigation, and protection of roads and highways (Brookes 1988; Bolton and Shellberg 2001). While there are numerous effects from the construction and maintenance of levees, one of the most dramatic effects is the isolation of parts of the floodplain from its main channel (Brookes 1988). 129 Monitoring Floodplain Restoration (a) (b) Main river habitat Braids Side channel Main channel Floodplain and terrace habitat Overflow channel Percolation channel Wall-base channel Figure 2. Up-valley oblique view of meandering river and wall-based channels (a), and examples of associated habitat types in and along main rivers (b). (Reprinted with permission from Peterson and Reid 1984.) One major consequence of floodplain alteration is the loss of habitats for fishes and other aquatic fauna (Beechie et al. 1994, 2001; Collins et al. 2002). For example, floodplain habitat isolation in several western Washington river basins has resulted in the virtual eradication of certain habitat types, such as large freshwater wetland and forest floodplain habitats in the lower portion of river basins (Beechie et al. 1994, 2001; Collins et al. 2002). Historically, most of the salmonid habitat (e.g., blind-tidal channels, side-channel sloughs, and beaver ponds) in two of these basins, the Skagit and Stillaguamish, were located in the floodplains and deltas (Beechie et al. 2001). These habitats have been reduced to less than 20% of their historic occurrence (Beechie et al. 1994, 2001). The disconnection of the floodplain not only disconnects habitat for fishes such as salmonids Pess et al. 130 Table 1. Descriptions of floodplain features. Note that categories are not mutually exclusive (e.g., wall-based channel is one type of side channel). Definitions modified from Allaby and Allaby (1999), with additional references noted below. Feature Description Floodplain A flat, depositional feature of the river valley adjoining the river channel formed under the present climate and hydrologic regime and during times of high discharge (Leopold et al. 1964) Alluvial rivers Channels formed in and by river-transported sediment under its current hydrologic and climatic regime (Leopold et al. 1964). An alluvial channel is self-forming in that its form reflects the load and discharge of the river rather than the constraints of bedrock. Meandering channels Channels that have a main flow (the thalweg) that oscillates from one side of the channel to the other. Features such as bars and pools become more fixed as the magnitude of the meander becomes greater (Leopold et al. 1964). Straight channels Channels with a thalweg that is situated in the same location and concentrates flows, bars, and pool formation. Naturally straight channels are relatively uncommon, and straight channel reaches rarely exceed lengths of 10 channel widths (Dunne and Leopold 1978). Braided channels Channel with transient bars dividing flow between multiple channels (Dunne and Leopold 1978). Sediment frequently is transported, and a mix of sizes, banks easily erode, and discharge changes rapidly (Reid and Dunne 1996). Anastomosing channels Channels that branch and depart from the main channel, sometimes running parallel for several kilometers before rejoining (Collins and Montgomery 2002). Geomorphic features between channels are more stable (e.g., islands) than those between braided channels Anabranching channels A distributary channel that departs from the main channel, sometimes running parallel for several kilometers before rejoining

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