Land use and climate change are two key factors with the potential to affect stream conditions and fish habitat. Since the 1950s, Washington and Oregon have required forest practices designed to mitigate the effects of timber harvest on streams and fish. Yet questions remain about the extent to which these practices are effective. Add in the effects of climate change—lower summer flow and warmer water temperatures in some streams— and more questions arise.
Scientists with the multipartner Trask Watershed Study set out to learn more about how the effects of climate change and timber harvests may interact and affect the long-term survival of cutthroat trout populations in the Oregon Coast Range. They collected data from four streams before and after an adjacent timber harvest and used that information to model stream and trout responses to these changing conditions. They also conducted experiments in semi-natural streams to evaluate the relationship of bird predation and instream cover to trout survival.
The scientists found that local variability in stream habitat, such as water depth and instream cover, play a greater role in reducing the effects of timber harvest and climate change on trout than previously realized. Instream cover and shade improve trout survival by providing a place to hide from predators.
Managers of the Prescott National Forest are obliged to evaluate the conditions of watersheds under their jurisdiction in order to guide informed decisions concerning grazing allotments, forest and woodland management, restoration treatments, and other management initiatives. Watershed condition has been delineated by contrasts between “good” and “poor” conditions (DeBano and Schmidt 1989). Good condition is characterized by vegetation and litter cover that is capable of absorbing precipitation, temporarily storing it, and slowly releasing it through a network of channels with minimal drainage density. Poor condition applies to areas where precipitation induces soil erosion and rapid sediment-laden runoff through an expanding network of channels. Evaluations of watershed condition face substantial challenges in attempting to determine a reference condition, the extent of departure from that condition, causes of that departure, and management actions that can return the watershed back toward the reference condition (McCammon and others 1998). These challenges are particularly great in watersheds of the arid and semi-arid Southwest, where flashy, sediment-laden runoff is a common natural condition.
Channel morphology has become an increasingly important subject for analyzing the health of rivers and associated fish populations, particularly since the popularization of channel classification and assessment methods. Morphological data can help to evaluate the flows of sediment and water that influence aquatic and riparian habitat. Channel classification systems, such as the one developed by Rosgen (1994) provide a useful shorthand for summarizing key morphological attributes of a river system. Accordingly, researchers have hypothesized that channel classifications could explain variation in native fish populations in rivers of the Southwest (Rinne and Neary 1997; Rinne 2005). Rosgen’s (1996) full methodology encompasses several levels of analysis arranged hierarchically from a general characterization of a stream basin to detailed measurements of channel change in specific reaches. The second and most popular level (Level II) of the Rosgen (1996) methodology provides a framework for categorizing stream reaches based on channel form and dominant substrate. While this classification is useful for describing variations in channel morphology, critics argue that it is less useful and perhaps even misleading for making inferences about channel condition and processes of development (Miller and Ritter 1996).
Streamside environments are inherently dynamic, yet streamside vegetation plays a key stabilizing role on riparian and aquatic habitats (Van Devender and Spaulding 1979; Van Devender and others 1987). Because of its dynamism, streamside vegetation is rarely the subject of classification analyses, yet it is a focal point for land managers regulating land uses, such as livestock grazing, that could potentially impact aquatic communities (Brown and others 1979). Livestock grazing along the UVR has been a politically charged issue, with recent years (1998 to present) witnessing a removal of livestock from the river corridor under Prescott National Forest management. However, livestock still graze on private lands, with some strays roaming onto State and Forest lands (see Chapter 2). During the same period, researchers observed declining populations of native fishes in the UVR, largely attributable to predation by introduced fishes (see Chapter 9), as well as to vast growth of woody plants post-1993 floods (see Chapter 2), and lateral erosion of historical terraces. Concomitantly, researchers have suggested that increases in woody streamside vegetation might be related to the cessation of livestock grazing (Rinne and Neary 1977; Neary and Rinne 1998, 2001a) as well as to other hydrological factors (see Chapter 2).
Degradation of streams is a threat to the recovery of the Apache trout, an endemic fish of the White Mountains of Arizona. Historic efforts to improve trout habitat in the Southwest relied heavily on placement of in-stream log structures. However, the effects of structural interventions on trout habitat and populations have not been adequately evaluated. We treated an actively degrading stream on the White Mountain Apache Reservation that harbored a unique population of Apache trout, using a combination of fencing to abate ungulate grazing, sedge transplants to speed recovery of degraded streambanks, and placement of rock riffle formations to stabilize the channel and restore aquatic habitat along 450 m of degraded stream. Following treatment, the bed refilled, water depth and width increased, and fine gravels (5-32 mm) became more abundant. Trout abundance in the stream declined as the degradation worsened, but it rebounded following the restoration treatments. While other factors, such as flooding and sampling methods may have influenced the fish populations, the results suggest that the treatment did not negatively impact the trout while preventing further deterioration of the habitat. The riffle formations were not observed to induce channel instability, cause excessive retention of fines, or raise barriers to fish passage, which are common hazards of conventional in-stream structures. This case study demonstrates that in specific ecological contexts, structural interventions may be appropriate for conserving native trout habitat.