Project Title: Inventory and Monitoring of Sediment Sources and Transport

Investigators: Mary Ann Madej, Tera Curren USGS-BRD Redwood Field Station Arcata, CA USA 707-464-6101 Ext. 5490 FAX: 707-822-8904 mary_ann_madej@usgs.gov Partners: Redwood National and State Parks Arcata, CA USA 707-464-6101
	Landslides are a major sediment source in north coastal watersheds, and the frequency of landsliding can be increased by road construction and timber harvest.

Project Description

Land use disturbances have caused accelerated erosion on steep forested hillslopes of northwestern California. The resulting loss of soil diminishes on-site productivity. The excess sedimentation in many north coastal rivers also results in destruction of pool habitat and spawning gravel quality. Through Public Law 95-250, Congress mandated studies of erosion and sedimentation in the Redwood Creek basin. The monitoring in this study is designed to track sediment from its sources (road failures, gullies, landslides, etc.) through tributaries and the mainstem of Redwood Creek in order to calculate where damage from sedimentation is still occurring or is likely to occur. Monitoring of streams draining watersheds of differing land use will show the effects of timber harvest, road construction and revegetation on sediment loads, runoff, peak flows, low flows and stream temperature.

This inventory involves mapping erosion features (landslides, earthflows, bank erosion, gullies and road failures) in the Redwood Creek basin and will quantify the amounts of sediment contributed to stream channels by these processes. A large flood in January, 1997 initiated or reactivated many erosional features. The Redwood Field Station analyzed 1997 aerial photographs to quantify changes in the spatial distribution, magnitude, and intensity of erosional processes from this flood.

The monitoring effort consists of evaluating channel stability (bank erosion, scour and fill, channel shifting) in streams and rivers. Particle sizes of channel bed substrate, sediment transport rates, water discharge and stream temperature are also parameters in the monitoring program.

Purpose

The purpose of the research is to establish a monitoring protocol to evaluate erosional problems, channel stability, and sediment transport in a highly erosive watershed. A knowledge of the type, size, timing and causes of erosion and sedimentation problems is the first step in prioritizing restoration projects, implementing improved land use practices, and preventing future damage. Linkages between erosional problems on the hillslope and damage to downstream anadromous aquatic habitat will be documented. National and state park managers, and adjacent private landowners will use this information in guiding future erosion control work in the Redwood Creek basin.

Progress/Results

A long duration but low intensity storm in 1997 caused relatively few road-stream crossing failures in the Redwood Creek basin in north coastal California, but initiated 365 landslides in a 720 km2 area. Shallow debris slides were the most common erosional process, but debris flows contributed more sediment to streams. Landslides were most highly concentrated in areas of previous failures (on steep inner gorge slopes), a trend that was also documented in the nearby Klamath Physiographic Province. In contrast to a large storm in 1964, the 1997 event in Redwood Creek produced almost an order of magnitude less material from mass movements. The 1997 landslides delivered about 720,000 tonnes of sediment to the Redwood Creek channel network, which represents 45% of the total sediment transport in Redwood Creek for the entire 1997 water year. The Redwood Creek channel locally aggraded 0.6 m, and pool depths and frequencies decreased following the 1997 flood.

Planned

Statistical analyses of landslide distribution and occurrence is being completed. Associations with road construction, bedrock geology, timber harvest and other variables will be tested.

Products

Curry, T. (In progress). Landslide frequency and mechanisms in the Redwood Creek basin, Northwestern California. MS thesis. Humboldt State University Department of Geology.

Madej, M. A. 2001. Development of channel organization and roughness following sediment pulses in single-thread, gravel bed rivers. Water Resources Research. Vol. 37 No. 8, p. 2259-2272.

Madej, M.A. 2003. Contrast in hillslope and channel response to large storms in Redwood Creek, California, USA. p. 71-80 in Proceedings of the International Workshop for “Source to Sink” Sedimentary Dynamics in Catchment Scale. Sapporo, Japan. 322 p.

Ozaki, V. L. and M. A. Madej. 1996. Long-term channel response to large floods and sedimentation. p. 137-140 in the Proceedings of the conference of Coast Redwood Forest Ecology and Management,. J. Le Blanc, ed. 170 p. Arcata, California.

Keywords: Landslides, gullies, erosion, sedimentation, timber harvest, aquatic habitat, sediment discharge, watersheds