Hydroelectric projects do affect the ecosystems of rivers and their surrounding areas. The degree, however, to which any one project affects a river varies widely. As discussed, one of the most important variables is whether a dam is part of a storage or run-of-the-river hydroelectric project. Other variables include the size and flow rate of the river or tributary where the project is located; the existing habitat and climatic conditions; the type, size, and design of a project; and whether a project is located upstream or downstream of other projects.
As changes in habitat occur, observation and time make it increasingly clear which plants, fish, and wildlife are affected. Some species end up doing quite well, others sharply or completely decline, and some are minimally affected.
The section Protection, Mitigation and Enhancement Strategies At Hydroelectric Projects reviews measures being taken to address continuing environmental impacts. It is important to remember these measures are part of a much larger and complex whole. As such, their perceived success or failure is often dependent on a number of non-hydroelectric project activities.
For instance, there are natural conditions that can dramatically affect the health of a river’s ecosystem and habitat. As an example, drought years at the beginning of the decade impacted critically important stream flows.
For salmon, the most important natural change relates to ocean conditions. Specifically, the effect called El Nino has resulted in higher sea levels and warmer surface temperatures. These conditions are being shown to greatly affect salmon survival and abundance.
Beyond these natural conditions are a host of man-made conditions. Such conditions include but are not limited to:
- Reduced streamflows from irrigation withdrawals and altered vegetation from land development, e.g., parking lots, roads, etc.
- Loss of riparian zone, changes in water temperature and quality, loss of large woody debris, erosion, and sedimentation can come from various types of logging practices, agricultural activities and other land uses, e.g., grazing of cattle, mining and the building of roads.
- Changes in estuary conditions from wetland drainage, diking, and navigational improvements.
- Introduction of hatchery fish (which now comprise over 75% of the salmon population) affecting the gene pool, viability, health, and abundance of “wild”salmon runs.
- The harvesting (catching) of fish by both commercial and sport fishermen.
While this section focused on impacts from hydroelectric projects, understanding how to maintain the health of rivers and tributaries throughout the basin requires investigating these much broader impacts as well. Additional sources of information on these impacts and what is being done to mitigate them include federal, state, local, and tribal agencies; public and private organizations that contribute to these impacts; and non-profit groups interested in these environmental issues.