No. 107 

A SIMPLE FILTER DAM FOR SMALL STREAMS 

Research foresters on the Fernow Experimental Forest 

were recently faced with the problem of conducting a logging 

job with "poor" skidroad standards on a 38-acre watershed 

above a municipal water-supply reservoir. 

To carry out the research assignment with a minimum 

of damage to the water supply, possibilities of removing or 

holding back the sediment after it got into the stream were 

investigated. 

In general, water supply people consider it impractical to remove sediment from natural streamflow because of 

the great fluctuations in flow and the fact that higher 

flows usually have higher turbidities. It is usually easier 

to treat only that part of the flow that actually enters the 

water mains. Thus solution of the Fernow problem could not 

be based on experience elsewhere. 

A number of different methods for filtering out the 

sediment were considered and tested on a small scale. As a 

result, four simple filter dams were installed in the lower 

reaches of the small stream, three above a weir pond and one 

below. Here the stream has a gradient of about 10 percent, 

and rather steep banks. 

The filter dams were constructed of small logs, 

posts, stock wire, and straw (figs. 1 and 2). First a log 

about 8 inches in diameter was placed across the stream from 

bank to bank about 3 feet above the streambed. Its ends 

were embedded in the banks. Then posts about 4 inches in 

diameter were placed against the crosslog on the upstream 

side, sloping down to the streambed at an angle of about 45 

degrees. The posts, spaced about 4 inches apart, were spiked 

to the crosslog. 
Figure 1. --Looking upstream at one of the filter dams. The 

dam is made of a crosslog, posts, stock wire, and straw. 

Stock wire was then laid on the posts. A 6-inch 

layer of straw was laid on this, and another piece of stock 

wire was laid on top. Tie wires were used to hold the posts 

and the two pieces of stock wire together. 

When completed, the filter dams were about 10 feet 

wide. In low-flow periods, water flowed through them; in 

high-flow periods it flowed both through and over them. The 

cost in material and labor was about $15 for each filter 

dam, not including the value of the small amount of timber 

cut at the site. 

During the logging operation, water turbidity of the 

stream measured as high as 5,000 turbidity units (a turbidity unit corresponds roughly to one part of soil per 

million parts of water). 

In the year since start of the logging job, the four 

filter dams caught a total of about 350 cubic feet of sediment. Most of this was trapped during the 2 months when 

logging was in progress. Though the filter dams trapped a 

considerable portion of the sediment, they by no means 

trapped all of it. However, the 350 cubic feet that was 

trapped amounts to an average of 70 parts of soil per million parts of water (by volume) of the total annual flow of 
the watershed, During stormflows much more than 70 ppm. was 

trapped; most of the time the catch was less. For a simple 

and inexpensive installation, this seemed a worthwhile 

accomplishment. 

Because this study was limited, it permits only conjectures as to the proportion of the total sediment caught 

by the filter dams, and as to final disposition of the 

trapped materials. Since the last filter dam downstream 

caught by far the smallest amount of sediment, additional 

filter dams probably would have added but little to the 

catch. Thus, for the type of sediment susceptible to screening by these filter dams, the proportion caught may have 

been considerable. Nevertheless, it was apparent that much 

sediment escaped. 

In time, the trapped materials may move downstream as 

the filter dams disintegrate. To avoid this, sediment could 

Figure 2. --One of the f i l ter dams as seen from above, 

showing the sediment trapped in the streambed. 
be shoveled out periodically; or possibly the sediment could 

be stabilized by seeding or planting. An alternative might 

be to construct the filter dams of more durable materials. 

Perhaps further research can develop more efficient 

devices than the one described here. If so, they might be 

useful in helping to safeguard downstream recreation and 

fish-habitat water values from logging-road sediment carried 

by small mountain streams. 

--K. G. REINHART 

Research Forester 

Northeastern Forest Experiment Station 

Forest Service, U. S. Dept. Agriculture 

Elkins, W. Va.