Saving Streams: Look to the Larvae


A good quality stream is not always obvious to the casualobserver. Murky, discolored water is one obvious indicator, but perfectly clear water may be badly polluted and devoid of life.

One might think that taking a sample of the water and chemically analyzing it would be a sure indication of the presence of pollutants. This might work in a pond, but in a flowing stream a rush of a toxic substance might not be detected at all if the sample were not taken at the time of the toxic release.

How then are we to know what is the condition of the stream on a continuous basis? Trout fisherman, who are vitally concerned with stream quality, know when the trout population is depressed, but why and how does this occur?

Scores of insects that live in and near streams produce larvae that live for a time in the water, most of them clinging to the undersides of rocks. They provide food for fish and other stream life. Some of them can exist only in the purest of water while others can tolerate a high degree of pollution. Entomologists (those who study insects) have shown that the presence of specific insect larval forms is a much better indicator of the overall and continuous water quality than attempts to measure other stream parameters. By examining the undersides of the rocks and identifying the larval species, one can obtain a reasonably good indication of the condition of the stream.

It is not necessarily chemical pollutants that degrade a stream. In a rapidly growing area such as Baltimore County, a major cause of trouble is the silting brought on by construction activities in the stream's watershed.

After a heavy rain, soil unprotected by vegetation washes down into the rushing stream. If it flows over a slow spot in the stream bed much of the silt will be deposited. Many of the streams in the southern part of the county have their beds greatly silted over. The rocks have been covered and the aerating riffles have disappeared. The streams have become wide and shallow. Few larvae can exist under the conditions and thus there are no fish. Devastating siltation can occur after a single heavy rainstorm.

Baltimore County must balance growth and its benefits with the residents' desire for clean, wholesome streams. A particularly fortunate conjunction of scientists, government and resident volunteers has begun an extensive survey of county streams, managed by an organization called Save Our Streams (SOS). Its purpose is to carry out a systematic survey of the insect life in the dozens of small and large brooks and streams throughout the county.

A rather rigid procedure is followed in order to provide the county with an accurate assessment of the present condition of a particular stream, thus helping to decide what sort of development is to be allowed and what the impact of construction in the stream's watershed might be.

The surveys depend on volunteers who, after a brief training session, go forth with their equipment to their assigned points, spending the better part of a Saturday wading, taking notes, turning over rocks in a systematic way, collecting the bugs, making tentative identifications and popping them into jars of alcohol. They also take careful notes on the surrounding habitat. Weeks later more volunteers gather in a lab at the University of Maryland Baltimore County to make final identifications with the help of professional entomologists.

The surveys are done three times a year: March, July and October. A surveyer may be pleased to find many stonefly larvae, a species found only in the purest of streams and providing food for trout. Or there may be no stoneflies, but many caddis flies, an indication of lessened quality.

It is an especially enjoyable activity for those who like to get out into the country. The areas visited are lovely and seeing them at three seasons of the year provides further appreciation of the beauties of Baltimore County. After several visits to the same spot, one develops a sense of propriety over the riffle in his stream. Destructive changes in the surrounding habitat are quickly noticed, sometimes with dismay.

Anyone who doesn't mind wearing boots or getting his feet wet is welcomed to take part in this program. Teaming up with an experienced volunteer makes the job easy. One can gain an appreciation of stream life, have fun in the country and do a service for the county all at the same time.

Olga Owens is a volunteer for Save Our Streams. More information about the organization is available by telephone at 969-0084.

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