Water from the Sierra Nevada has played a critical role in the development of California since the discovery of gold in a channel leading to a water-powered sawmill in 1848. Development of streams and other resources of the Sierra Nevada over the past 150 years has met the demands of downstream society but has impaired the quality and availability of water for both ecological and social needs in many parts of the mountain range. Because of the extensive network of dams and diversions, few river systems in the range have natural flow regimes over much of their length. In most river basins, this active management of the water itself affects the annual water balance, temporal distribution, flood hydrology, minimum flows, and water quality much more than any impacts on the landscape. Ironically, the primary benefits to society of water from the Sierra Nevada cause the primary impacts.

Compared to the intentional alteration of streamflow through water management, hydrologic side-effects of changes in land use are difficult to measure but are still believed to be significant. Major changes in water and sediment regimes have not been observed in the main rivers and their larger tributaries as a result of shifts in land use. There may be a signal, but it is not obvious or well-quantified. Hydrologic changes resulting from land management are most likely to be found in headwater areas where a large fraction of the catchment has been affected. Diversion of water from a stream will limit transport of excess sediment loads and thereby compound the impacts of land disturbance. Roads are believed to have increased sediment yields substantially, but the inferred changes have not been measured in the Sierra Nevada. Overgrazing has probably altered channel conditions extensively, but the scarcity of ungrazed reference sites limits ability to quantify impacts. Rapid expansion of foothill communities has theoretically altered runoff and erosion processes enough to cause noticeable impacts in downstream channels, but quantitative and documentary evidence outside of the Tahoe Basin is lacking. Conversion of forest lands to roads associated with timber harvesting may have increased annual water yields and peak flows somewhat at the small watershed scale (see Marvin article in this issue). However, decades of successful fire suppression may have increased evapotranspiration relative to a pre-1850 fire regime and partially compensated for the flow increases attributed to roads and harvests. The offsetting magnitudes of either impact cannot be quantified at this time. The legacy of fire suppression creates substantial risks of serious hydrologic impacts from potential conflagrations.

Overall, chemical water quality remains high, but cannot be considered pristine. Because of widespread biological contamination, surface waters throughout the range cannot be assumed to be drinkable. A few local problems are very serious: Lake Tahoe, some abandoned mines, and some communities. Quality of receiving waters from the larger cities in the foothills has been degraded. Excessive sediment production is the most widespread nonpoint-source problem, but its extent and severity are unknown. Studies in other areas suggest that roads are the overwhelming source of sediments than end up in wildland streams. Disturbance in and near stream channels generates the vast majority of sediment transported by the streams. Existing information about sediment yields in Sierra Nevada rivers is largely obsolete, and new reservoir sediment surveys are necessary to determine whether changing land use has accelerated sedimentation in the past few decades. Because of the importance of flowing water in diluting and dispersing pollution, alteration of streamflow by storage and diversion may be the fundamental water quality problem in the Sierra Nevada.

Related SNEP chapter:
Kattelmann, R. Hydrology and water resources. Vol. 2, chapter 30, pp. 855-920.

Riparian Systems
Riparian areas of the Sierra Nevada were considered to be one of the most ecologically and politically sensitive topics of the SNEP efforts. A draft of an initial report on riparian conditions (Kattelmann and Embury) was blasted in the external review process, and a second report (Kondolf et al.) was commissioned to address the reviewers' concerns with the first report. Riparian areas are the focal point of many resource conflicts in the Sierra Nevada because they are a critical ecological link between land and water. They have been extensively affected by direct removal or inundation of riparian vegetation and by alterations to the conditions on which the riparian vegetation depends.

Unfortunately, the field data base necessary to properly assess the health of riparian areas throughout the Sierra Nevada does not exist. However, from the extent of human activities known to impact riparian areas, we can infer substantial impacts. Moreover, map and aerial photograph analyses of a large sample of Sierran watersheds show that virtually all riparian corridors are interrupted by gaps caused by such human activities such as construction of road or railroad crossings, human settlements, dewatering of streams, grazing, timber harvest, and mining. The largest gaps are caused by reservoirs, many of which exceed 0.5 km (0.3 mi) in length. About 1,000 km (600 miles) of riparian corridors have been submerged under reservoirs throughout the Sierra Nevada. These reservoirs and other gaps break the continuity of the riparian corridors and impair wildlife migration.

The basic functions of riparian systems, such as providing shade, stability, and organic matter to streams and habitat for avian and terrestrial wildlife, still remain in most places although often in impaired form. In addition, dramatic loss of functions is evident in mountain meadows throughout the Sierra Nevada. In meadows and along particular stream reaches, there are thousands of localities needing restoration of riparian functions. For channels below reservoirs, dedicated management is needed to provide occasional high flows to mimic hydrologic effects of natural floods in maintaining riparian vegetation.

Establishing riparian management zones of adequate width is probably the single most effective strategy for protection and maintenance of the ecological values of riparian areas. Vegetation removal and ground disturbance should be strictly controlled in these zones, both to preserve the riparian habitat itself and for its beneficial influence upon aquatic habitat. Both federal and California forest practice standards or rules specify restrictions and practices in riparian areas intended to protect streams and moderate disturbance from land use (Moyle et al. and Menning et al.). The main issues are not about the special nature of riparian areas but rather how much area belongs in this category and what activities are acceptable. Ecological functions and process should be guides to use and protection. Criteria for establishing riparian management zones based on ecological concepts were developed by the SNEP team. Riparian ecological functions and physical processes take place in three areas at varying distances from the aquatic system: a community area, an energy area, and a land-use influence area (Erman et al.). The size of these areas depends on the local characteristics that define them. Shifting to a recognition of the community, energy, and buffering requirements of riparian areas will aid in protection and management of the entire riparian system.

Related SNEP chapters:
Erman, D. C., N. A. Erman, L. Costick, and S. Beckwitt. Management and land use buffers. Vol. 3, appendix 3 to chapter 5, pp. 270-273.
Kattelmann, R. and M. Embury. Riparian areas and wetlands, Vol. 3, chapter 5, pp. 201-273.
Kondolf, G. M., R. Kattelmann, M. Embury, and D. C. Erman. Status of riparian habitat. Vol. 2, chapter 36, pp. 1009-1030.
Menning, K. M., K. N. Johnson, and L. Ruth. A review of current non-federal policies on non-federal lands in the Sierra Nevada that affect aquatic, riparian, upland, and late-successional biological diversity. Addendum, chapter 1, pp. 1-32.
Moyle, P. B., R. Kattelmann, R. Zomer, and P. J. Randall. Management of riparian areas in the Sierra Nevada. Vol. 3, chapter 1, pp. 1-38.

Cumulative Watershed Effects
The combined or cumulative effects of human activities on aquatic communities and attributes of water garnered much attention throughout the SNEP efforts. The evaluations of water resources (Kattelmann) and biotic integrity of watersheds (Moyle and Randall) were largely assessments of the consequences of 150 years of resource development on aquatic resources of the Sierra Nevada. The case study of Lake Tahoe (Elliott-Fisk et al.) illustrates how a wide variety of local disturbances combined to alter the nutrient balance of the lake and degrade one of the best-known treasures of the Sierra Nevada.

Because the SNEP team recognized that continuing and forthcoming impacts to the hydrologic system of the Sierra Nevada will have to be evaluated in terms of their contribution to the effects of other past and present disturbances, existing methodologies were assessed and new procedures were developed. A dozen different approaches to analyzing cumulative watershed effects (CWE) were reviewed by Berg et al. Because of the lack of data sets to objectively evaluate the results of a cumulative watershed effects analysis, a group of desirable attributes of a CWE procedure for application in the Sierra Nevada was developed to provide the reader with a basis for comparing different techniques. The chapter by Berg et al. recommended that some adaptation of the watershed-analyses packages by the state of Washington and the Pacific Northwest region of the U. S. Forest Service might be the most appropriate approach for establishing broader context for subsequent detailed CWE assessments. The screening model developed by Costick as part of SNEP contributes to CWE analysis by identifying sites with high potential for delivering sediment to streams.

The SNEP policy-analysis team required an automated method of accounting for CWE within their models of alternative land management scenarios and their consequences. This group modified the equivalent roaded area (ERA) methodology of the California Region of the Forest Service for their modeling exercises (Menning et al.). Members of the SNEP team focusing on aquatic resources had suggested separate accounting of disturbance and limits on disturbance as a function of distance from streams. The model described in Menning et al. calculates ERA values and compares them to ERA limits in each of three zones: inner riparian, outer riparian, and uplands. The widths of the riparian zones are determined by the concepts of community, energy, and land influence regions described by Erman et al. This addition of explicit spatial relationships to accounting of cumulative watershed effects should be a useful improvement in estimating consequences of land management activities.

Related SNEP chapters:
Berg, N. H., K. B. Roby, and B. J. McGurk. Cumulative watershed effects: Applicability of available methodologies to the Sierra Nevada. Vol. 3, chapter 2, pp. 39-78.
Costick, L. A. Indexing current watershed conditions using remote sensing and GIS. Vol. 3, chapter 3, pp. 79-152.
Elliott-Fisk and 13 others. Lake Tahoe case study. Addendum, chapter 7, 217-276.
Erman, D. C., N. A. Erman, L. Costick, and S. Beckwitt. Management and land use buffers. Vol. 3, appendix 3 to chapter 5, pp. 270-273.
Kattelmann, R. Hydrology and water resources. Vol. 2, chapter 30, pp. 855-920.
Menning, K. M., D. C. Erman, K. N. Johnson, and J. Sessions. Modeling aquatic and riparian systems, assessing cumulative watershed effects, and limiting watershed disturbance. Addendum, chapter 2, pp. 33-51.
Moyle, P. B. and P. J. Randall. Biotic integrity of watersheds. Vol. 2, chapter 34, pp. 975-985.