In addition to the effects on instream flows on aquatic resources discussed in Section 4.1, project operations that affect instream flow regimes can have various effects on riparian vegetation communities and on special-status and non-native invasive plants. These include:

• impairment of natural riparian vegetation dynamics, including effects on natural recruitment and woody vegetation encroachment into the channel,
• drowning or desiccation of special-status plants, and
• introduction or spread of non-native invasive plant species.

After gathering basic data on species composition and structure, as described above, the approach to assessing potential effects is to evaluate “indicators” of health and compare them with a reference condition. The following indicators are often assessed:

• species composition and age or size structure,
• longitudinal connectivity of the site,
• width of the corridor (which includes an assessment of the level of encroachment), and
• bank erosion.

In order to assess some of these indicators, more focused survey efforts are sometimes performed. For instance, in order to assess age structure, a sample of trees are often cored. To assess levels of encroachment, a time series of photographs can be analyzed. Additionally, hydrological and geomorphic conditions need to be assessed. These conditions are most often surveyed as part of a separate study for the same project; results are then incorporated into analyses of effects to riparian vegetation.

After selecting and assessing the status of a few parameters, the reference condition is chosen. It can either be a site nearby that is relatively undisturbed (e.g., reach that isn’t dammed), or can be a description of riparian ecological “health” obtained from the literature. Unfortunately, a good reference condition is not always available. Often, all reaches in the nearby, relevant area have been regulated or otherwise altered by human activities. An important complicating factor is that natural riparian communities vary with factors such as channel gradient, valley width, and lithology, which means the appropriate reference state is variable and dependent on site-specific conditions. This natural heterogeneity in riparian communities makes it even more difficult to define an appropriate reference condition, regardless of whether it is based on a nearby unimpaired reach or the scientific literature. Therefore, analyses and syntheses of measured indicators and conclusions derived about indicator’s health (e.g., that there is encroachment beyond what would be expected in an uninhibited “natural” setting) often must rely on professional judgment.

The following references are recommended for additional information on ground disturbance in relation to special-status plants, lichens, and fungi and plant communities:

Harris, R. R., C. A. Fox, and R. Risser. 1987. Impacts of hydroelectric development on riparian vegetation in the Sierra Nevada region, California, USA. Environmental Management 11: 519-527.

Harris, R. R. 1989. Riparian communities of the Sierra Nevada and their environmental relationships. Pages 393-398 in D. L. Abell, editor. Proceedings of the California riparian systems conference: protection, management, and restoration for the 1990s. General Technical Report PSW-110. USDA Forest Service, Pacific Southwest Forest and Range Experiment Station, Berkeley, California.

Harris, R. R. 1999. Defining reference conditions for restoration of riparian plant communities: examples from California, USA. Environmental Management 24: 55-64. Jansson, R., C. Nilsson, and B. Renofalt. 2000. Fragmentation of riparian floras in rivers with multiple dams. Ecology 81: 899-903.

Jansson, R., C. Nilsson, M. Dynesius, and E. Andersson. 2000. Effects of river regulation on river-margin vegetation: a comparison of eight boreal rivers. Ecological Applications 10: 203-224. Naiman, R. J., H. Decamps, and M. E. McClain. 2005. Riparia: ecology, conservation, and management of streamside communities. Elsevier Academic Press, Burlington, Maine.

NRC (National Research Council). 2002. Riparian areas: functions and strategies for management. Committee on Riparian Zone Functioning and Strategies for Management, Water Science and Technology Board, National Research Council. National Academy Press, Washington, D. C.

Rood, S. B., and J. M. Mahoney. 1990. Collapse of riparian poplar forests downstream from dams in western prairies: probable causes and prospects for mitigation. Environmental Management 14: 451-464.

Rood, S. B., C. R. Gourley, E. M. Ammon, L. G. Heki, J. R. Klotz, M. L. Morrison, D. Mosley, G. G. Scoppettone, S. Swanson, and P. L. Wagner. 2003. Flows for floodplain forests: a successful riparian restoration. BioScience 53: 647-656.

Ward, J. V. and J. A. Stanford. 1995. The serial discontinuity concept: extending the model of floodplain rivers. Regulated Rivers: Research and Management 10: 159-168.

5.1 Ground Disturbance

Page 84 of 134

5.3 Impoundment of Water