Impacts of Development

Agriculture in the Pajaro River Valley.

An estimated 95 percent of riparian habitat has been lost in California during the last century (RHJV 2000), resulting in significant loss of salmonid habitat and biological diversity in general. California rivers once meandered across their forested floodplains, overflowing their banks with winter rains to create a complexity of habitat types, but they have been severely degraded and simplified since European settlement. Floodplain development and the straightening of creeks for commercial, residential, and agricultural activities have reduced the width of the riparian zone and, accordingly, the amount and quality of riparian habitat (see the Land and Resource Uses section for more information). When streamside vegetation is removed, light input tends to increase, large organic debris input decreases, and sediment and nutrient loads from the watershed increase. This may cause water temperatures to rise and may favor the growth of algae, resulting in a lowering of dissolved oxygen levels. Reduction or loss of riparian cover has reduced fish populations in some stream ecosystems by as much as 80 percent (Gore and Shields 1995).

In the Aptos Creek Watershed, local studies have been conducted to map riparian corridors and invasive non-native species. Water quality and sedimentation monitoring is also being conducted to help support local habitat restoration planning.

Removal of riparian vegetation may have substantial impacts on hydrologic and geomorphic processes. In natural systems, standing and fallen trees and shrubs in the riparian zone hinder the flow of floodwaters, substantially reducing their velocity and causing sediment to be deposited, eventually forming hummocks and terraces. Large root masses of living trees anchor soil and reduce bank erosion. Fallen logs (large woody debris) form pools within the channel of small streams and serve to trap sediments and to dissipate stream energy. Reducing or removing riparian vegetation disrupts these natural functions and can result in changes to physical processes such as sediment transport, channel width, and stream configuration.

Landowners and community members are increasingly committed to conserving, enhancing, and restoring riparian areas while still maintaining a viable economy. Riparian vegetation provides numerous benefits, both by maintaining stream conditions – which protects land – and as habitat for natural communities. Restoring and preserving riparian areas is increasingly recognized as a win-win for human interests and ecosystem function.

Identifying areas for enhancement or restoration is a first step towards improving riparian function. Because most vegetation mapping is currently done on a large scale (see Upland Plant Communities), riparian mapping is usually completed at the stream reach level to identify areas of concern. Information about riparian corridor width, species composition, successional stage, large woody debris, and invasion by non-native species is valuable for resource managers to make decisions regarding riparian health (see Planning and Management for more information).

References

Arnold, C. 1990. Wildlife corridors. California Coast and Ocean Magazine, California State Coastal Conservancy (Summer):10-21.

Gore, J.A., and F.D. Shields. 1995. Can large rivers be restored? Most restoration projects are only attempts to rehabilitate selected river sections to a predetermined structure and function. BioScience 45 (3):142.

Riparian Habitat Joint Venture (RHJV). 2000. "Version 1.0. The Riparian Bird Conservation Plan: A Strategy for Reversing the Decline of Riparian Associated Birds in California." California Partners in Flight. View on-line document .