Cross-section survey at Salyer from the Long Term Trend Monitoring Project for the South Fork Trinity River Watershed

Excerpts from Dresser, A. T., C. Cook, and M. Smith. 2001. Long Term Trend Monitoring Program for the South Fork Trinity River watershed. U.S.D.A. Forest Service, Six Rivers National Forest. Eureka, CA. 89 pp.

"Repeated cross section surveys are the simplest way to quantify changes in stream channel geometry. The protocol is well established, cost is relatively low, and results are easy to evaluate.

The repeated surveying of established cross sections is a primary tool for long-term trend monitoring. The data provided can be used to calculate mean bed elevation, bankfull width, mean depth at bankfull, and the width-to-depth ratio. All these parameters can be tracked over time to illustrate changes in stream morphology. The bankfull channel is used for these calculations because well-defined relationships exist between the bankfull channel and hydrologic variables such as discharge of water and sediment. The elevation of bankfull discharge is determined in the field at the time of survey.

Bankfull width is simply the width of the water surface at bankfull discharge. In alluvial channels, bankfull width would be expected to increase when the sediment supply exceeds transport capacity. Because the South Fork Trinity River is typically confined within narrow valley walls, bankfull width is not expected to change much.

Mean water depth is calculated by taking the area under the bankfull water surface and dividing by the bankfull width. Mean bed elevation is the elevation of the bankfull water surface minus the mean water depth. By definition, the mean bed elevation determines whether the streambed is aggrading or degrading, and is a clear representation of trends in sediment storage. The cross section at the USGS gage near Salyer (11529000) is an example of how repeated surveys can show channel filling and the subsequent erosion and return of the streambed to pre-flood levels.

The width-to-depth ratio is the width of the bankfull water surface divided by the mean water depth at bankfull discharge. Streams are predicted to get wider and shallower as they become overwhelmed by sediment supply in excess of transport capacity (the width-to-depth ratio will increase).

Cross sections were surveyed at five or more representative riffles in each of the four reaches. Cross sections may be surveyed with a tape, level and stadia rod, or laser level. Cross sections targeted riffles which are more sensitive to sediment supply than pools (Lisle, personnel communication) and were located away from the influence of wood or other temporary structures affecting channel geometry.

Cross sections have some limitations. They only represent one place in the channel, although multiple cross sections help to define a longer reach. Also, cross sections represent one moment in time and do not show changes that may have occurred during high flow events such as scour and fill. Finally, changes in cross sections are not easily related to fish abundance or survival.

Reach 4 at Salyer (river mile 1.5 to 6.2) is the most downstream monitoring reach, extending from river mile 6.2, just downstream of the USGS gage near Salyer, to Sandy Bar. This reach should reflect changes in the entire watershed. In addition, it has a lower gradient (0.2 percent) and contains more stored sediment than any other reach below Hyampom Valley. Thus, it is the most sensitive reach of mainstem channel in the lower watershed. Some historic cross section data associated with the USGS gage near the Canclinni property show 20 feet of aggradation after the 1964 flood and subsequent degradation to near 1955 levels. It is possible that the lower mainstem has already flushed through sediments from the 1964 flood. More comprehensive monitoring of the Salyer Reach may reveal if the changes at the USGS gage are anomalous or actually represent lower mainstem conditions.

Little data presently exist regarding the condition of pools in the Salyer Reach although anecdotal reports from long-time local residents suggest that pools were much deeper before the 1964 flood (Berol, 1995). Because this reach is used by nearly all anadromous fish in one way or another, it is an important place to monitor.

The channel in the Salyer Reach is quite confined, often by bedrock walls. Average bankfull width is 290 feet. Streamside landslides are common in this reach, including several large features. Although landslides may cause local data anomalies, large active landslides are common throughout the lower South Fork Trinity River."