Chapter 8: Monitoring and Evaluating Instream Habitat Enhancement
Philip Roni, Andrew H. Fayram, Michael A. Miller
The placement of physical structures into lotic environments to create pools, to alter channel morphology, and to provide cover and habitat for fish and other aquatic organisms has a long history (White 1996, 2002). It was one of the first methods used to mitigate habitat degradation and to increase fish production in streams and rivers (Tarzwell 1934, 1937, 1938), and also is arguably one of the most common and widespread restoration methods in regular use throughout North America and Europe. Many different configurations of instream structures and methods have been used over the years to improve habitat (White and Brynildson 1967; Vetrano 1988; Hunt 1993; Riley and Fausch 1995), but they are all generally composed of rocks, boulders, trees, and brush bundles. They can be categorized by purpose (e.g., create pools, trap gravel) and material and can include such structures as boulder or log weirs, dams and deflectors, cover structures (particularly common in streams of the midwestern states), rootwads and brush bundles, spawning pads, gabions, and, more recently, the construction of logjams in larger rivers (Table 1; Figure 1). These techniques, generally referred to as instream restoration or enhancement, involve placement of materials into the active stream channel or actual manipulation of the active channel itself in an effort to improve fish habitat (Table 1). Because these activities seek to enhance habitat rather than restore a deficient process (e.g., riparian, hydrology) or return a stream to some predisturbance state, they are technically habitat enhancement, and we refer to them as such in this chapter.
Most instream enhancement techniques used today were developed in the 1930s in the relatively low-gradient streams of the upper Midwest (Hunt 1993; White 1996). They were designed to enhance trout habitat that had been simplified and degraded through agriculture, forestry, and other land use. The primary purpose of instream structures in midwestern streams was to narrow channels, to provide cover for trout and other game fish, and to reduce mortality due to predation (Tabor and Wurtsbaugh 1991; Gregory and Levings 1996). Instream enhancement techniques developed in the Midwest were later applied to streams in the western United States with varying degrees of success (Ehlers 1956; Armantrout 1991). They subsequently were modified for use in the higher energy mountainous streams common in western North America, where the objectives focus primarily on creating pools and on increasing habitat complexity, rather than providing cover (Reeves et al. 1991). Comparable instream enhancement efforts have been applied in Western Europe, which has experienced similar yet more intensive degradation of watersheds due to a longer history of land use (Iversen et al. 1993; O’Grady 1995; Cowx and Welcomme 1998; Gortz 1998; Laasonen et al. 1998).