Over the past 15 years, fisheries biologists have been using instream structures to restore or enhance fish habitat in streams. In California funding for instream structures has been made available through a series of legislation and State bond initiatives. Funding is usually earmarked for use on materials and labor only, and design and monitoring of these projects was left largely unfunded. As a result, many instream structures have been placed into streams but little is known about their effects on stream morphology or aquatic communities.

Six Rivers National Forest, in cooperation with the California Department of Fish and Game and the Pacific Southwest Research Station has undertaken several short-term, intensive studies of instream structures.

Over the past 12 years about 1200 instream structures have been placed into anadromous fish streams throughout the Forest. In 1989 we began a forest-wide long-term monitoring program of all instream structures in an effort to learn more about how different structures affect streams, so that we can make more effective use of fishery restoration monies.

Both the physical and biological effects of the structures are monitored in the project areas before and after project implementation. Once project sites are identified, the entire area that will be affected by the structure(s) is mapped using a Redy-Mapper & Trade; (a quick mapping tool) that produces a fairly accurate planar map of the area. For structures such as deflectors, boulder clusters, weirs, or digger logs that are intended to create localized changes in channel morphology we shoot a longitudinal profile of the area and cross sections through locations that are intended to be modified. These data yields three-dimensional documentation of the physical attributes of a site before instream structures are installed.

For structures that are only intended to increase cover only planar maps are constructed. The same data set is collected after the structures have been placed and sufficient flows have occurred.

These data will be collected multiple times in future years to document the evolution and effective life of various structure types placed in different channel types. The planar maps, longitudinal profiles, and cross sections are entered into DesignCad software that enables maps to be stored on different layers and then overlayed. Pre-project maps and profiles can then be overlayed with post-project maps and profiles to detect the effects of structures.

Biological monitoring of instream structures varies with the intent of the structure and the fish species occurring in the area. Instream structures on the Six Rivers National Forest are targeted to create or improve habitat for salmon and steelhead during rearing and/or spawning phases of their life cycles. In most cases few, if any, other fish species occur in our streams.

For streams containing chinook salmon populations, spawning index reaches are surveyed repeatedly every 7-10 days during spawning season (October through January). Structures such as weirs, deflectors, and boulder clusters sort and collect gravels suitable for chinook salmon spawning. The amount of spawning associated with these structures is documented during spawning surveys.

In some streams spawning associated with structures regularly comprises more than half of the spawning within our index reaches. The main intent of most structures on the Six Rivers is to increase depth, cover, and habitat complexity for both spring rearing of chinook salmon juveniles and late summer rearing of one and two year old juvenile steelhead. If habitat quality is increased it follows that fish utilization of these areas should increase.

Fish counts are conducted in structure reaches as many years prior to project construction as possible and then periodically after the structures are installed. Monitoring of these structures is usually done by direct snorkel-observation fish counts through the reach affected by the structures. Paired divers start at the downstream end of a structure reach and work slowly upstream counting all fishes in the reach. Association with specific structures is noted. In conjunction, nearby control reaches are also counted on the same day. Because natural year-to-year fluctuations in fish populations are great, control reaches are necessary to separate the effects of structures from the effects of natural variation in population size.

Chinook salmon counts are conducted during the spring (April through June) and steelhead counts are conducted during summer low flows (August through September). Due to uncertain stream conditions during spring dives we have had only limited success counting juvenile chinook salmon. The success of this monitoring program will be determined over time.

We hope to be able to increase our knowledge of the effectiveness of various structure types and use this knowledge to refine instream structure technology.

Alteration of stream habitat can affect other members of the stream community. We have some information now on the effects of boulder deflectors on frogs and we hope to gather a broader range of data on specific structures in the future.

Dave can be reached at (707) 442-1721