Caring for and Healing the Earth


The Role of Streamside Vegetation

By Dana Griffin III

(reproduced from Colorado Streamside, Colorado Trout Unlimited, Winter 1984)

So much has been written on what is re­quired to maintain healthy populations of trout that to get into any aspect of the subject places the reader at some risk, that of being dragged through a set of facts which he probably already knows. Nevertheless, some things are sufficiently important that we run a greater risk in not repeating them every now and then.

Take, for example, the role that streamside vegetation plays in support of trout. If there is any subtlety to this topic, it may lie in the carnivorous habit of trout, a life style that may be hard to relate, at least on first reflection, to the flora on the bank. If trout grow fat and sassy on a diet of insects, spiders, frogs, and other fish, what, then, has vegetation to do with it? The answer, as it turns out, is plenty.

A substantial number of aquatic insects on which trout depend are detritus feeders (although some, like the nymph of the dra­gonfly, are carnivorous). Detritus, for the most part, consists of the partially decomposed remains of plants. This mater­ial derives from a variety of sources -- ­from aquatic plants and from streamside vegetation, the foliage of which often drops into the water.

Several other groups of aquatic organ­isms that fit into the food web, leading eventually to trout may also feed on de­tritus or on the bacteria which decompose dead plant tissues. These groups include rotifers, snails, and some bottom-feeding fish. Removing or even reducing the normal amount of streamside vegetation strikes a blow against trout by endangering the support base on which trout (and other members of the stream community) depend. Paranthetically, one of the devastating con­sequences of acid precipitation, in addi­tion to interfering with gas-exchange by gill-breathers, is to reduce the produc­tivity of many kinds of aquatic plants (although a few, like sphagnum moss, may actually flourish in an acidified environ­ment) or those growing nearby the water system. This has a reverberating impact right on up through herbivores reaching and extending beyond the trophic level of trout.

Another life-support role played by ri­parian vegetation vis-a-vis trout is in buffering streams against detrimental run­off. As we should be aware by now, not all run-off water reaching streams is healthy for aquatic life. Some run-off carries high loads of silt. Silt, in sufficient volume, threatens trout by clogging gill membranes, by burying trout eggs to depths that they die of oxygen deficiency and by killing nymphs.

If the organic component of silt is high enough. this may raise the BOD (bio­logical oxygen demand) of the water through the utilization of oxygen by bac­teria and fungi which feed off the organic materials. Given high enough BOD levels, trout will either abandon the section of the stream so affected or will simply suf­focate for lack of an adequate supply of oxygen.

If all that weren't enough, today run­off may well carry a list of other alien compounds that are not good for trout or the aquatic ecosystem in general. Pesticides, herbicides and other synthetic com­pounds do not remain on the original sites where they are applied. Some portions reach trout streams where they may contin­ue to do what they were made to do -- kill living organisms.

Land-clearing and road-building may im­pact trout streams in a negative way by not only increasing the silt load of those streams but also by acting as a conduit for toxic compounds, compounds which may have been used originally for entirely reasonable reasons, but in an entirely different context. Tests have shown that rain or snow landing on the hardtop sur­face of a road will contain measurable quantities of petroleum residues, includ­ing oils, greases and tars. No case has been made or can be made for any of these ingredients helping trout. If placed in streams in sufficient amounts (and the toxic threshold may be alarmingly low), they can only be detrimental to the fish we hope to nurture and support.

A key word in all of this is moderation. Can man moderate his activities so that upland and aquatic communities of plants and animals are not threatened? His his­tory up to this point is not unblemished, but he does have the capacity to learn and change his ways. Some silting of streams, for instance, is perfectly natural. Spring and early summer run-off from melting snowbanks swells streams and increases their turbidity from melt waters cascading down over talus slopes and bare soil. Pre­suming the snow is not heavily contaminat­ed with acid particles (not an entirely safe presumption nowadays), this kind of process scarcely threatens stream life. Stream systems have developed over mil­lions of years in environments where this cyclical process has always been a factor. Spring and summer silt is moved downstream as the season progresses and as the fre­quency of rain diminishes. The streams are given time to clean their bottoms before the cycle begins again. A wholly, (or un­holy) different kind of silting problem is presented when a major new road is carved through the countryside or large amounts of forestlands are cleared. These changes in the landscape create an enormous and long-term supply of silt which may be de­livered to streams in enough quantity as to overwhelm the cleansing capacity of the aquatic system.

Streamside vegetation can retard the arrival of silt and its often toxic components, helping to maintain of water quality.

Still a third role played by the plants associated with trout streams is that of supplying a variety of terrestrial insects and spiders on which trout feed. Though there is hardly a fly fisherman around who has not cursed an alder or willow thicket for "grabbing" his or her fly, thus ruin­ing a perfect cast, these same individuals, queried in a more reflective moment, sure­ly would admit that alders, willows and other woody streamside plants are perform­ing a unique service. Grasshoppers, of which the Rockies have seen quite a few these past summers, not infrequently end up in the gullet of trout after their un­gainly flight has taken them across stream only to bang into a high-silhouetted cot­tonwood or willow and then to land onto the water in full view of lurking trout. Think of the number of ants, caterpillars, beetles, and a myriad menu of other leggy items that climb stems and branches along streams, positioning themselves perfectly, from a trout's point of view, for delivery to an eager clientele. All that is requir­ed is a slight misstep, a puff of wind. or a vibrating jolt from a swaying branch) and the meal is served up.

Before another trout season is in full swing, wouldn't it be appropriate for all lovers of trout fishing to consider the rewards of good stream stewardship? Among those activities easiest to carry out, but with long-term benefits to all, is the careful husbanding of our streamside plant life. Readers, individually, or in groups, could assume responsibility for sections of specific streams.

That responsibility would begin with making surveys of bank conditions. Those areas found to be denuded of plants or which seem vulnerable to erosion can be targeted for restoration. Some attention to basic habitat requirements of plants will help. In flat, sunny stretches, sed­ges, rushes, or willows can be transplant­ed. Planting "starters" the diameter of a 2-pound coffee can at one foot intervals could spell the difference later on in whether these sections of bank hold up or erode materials into the stream. Along steeper and/or shadier areas, other plants, like alders, can be established. Banks that are in an especially bad state may require a more substantial effort includ­ing shoring up with logs and rocks.

Let's be honest. Organizations like Trout Unlimited were not begun because the country is overflowing with crystal clear streams brimming with trout 18 inches long. If there is a trout fisherman left who be­lieves that good, productive trout fisher­ies are an inalienable right guaranteed by Washington, Holy Writ or both, then tell that poor soul to wake up! If we want to keep trout in our streams, we will have to work for it.


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