In my trout fishing, I have come to realize how useful and important snags are--not only for finding trout, but for their function in the total river system. A snag is a tree, limb, or large bush that obtrudes into the streamflow. A snag may be small, as in the case of a single limb or bush that enters the streamflow, or it may be large involving one or more very large trees. Snags may lean into the water or they may be trees that have fallen all the way into the stream. Some trees that fall into the water may remain attached to the bank or may be detached. The effects of the snag will vary with its size, the type of tree, its relation to the bank, and its position in the streamflow. The generation of snags--the progress of a tree from a sapling to a mature tree to a snag--is an interesting and somewhat complex phenomenon in stream ecology. The conventional wisdom holds that snags are generated when a flood or some other disturbance dislodges a tree--i.e., that the snag is the product of some specific erosive work. Snags, however, are a general--typical--feature of rivers having wooded watersheds, and snag generation is a function of erosive events but also of the natural dynamics of streams [particularly their capacity to meander], and is related to the activities of many mammals and birds as well.
Far less obvious but perhaps the most important factor in the generation of snags is the effect of light falling upon the river. Within a mature and undisturbed tract of forest, trees tend to even out in height and form a canopy. Seedlings germinating under this canopy tend to grow directly toward the light and on level sites, this tendency is straight upward, resulting in fairly spindly, but straight, saplings under the mature trees. At the edge of the forest, or where the forest has been disturbed as in a fire or by building a road or in a clearing for a house or field, the edge area receives more light and growth is accelerated along the edge for those seedlings which generate. A river which flows through a forest creates a double edge within the forest area where it passes. This allows more light to enter this part of the forest than enters the area further from the bank. The seedlings which germinate grow toward this light. The site for these seedlings is different, however, from that of seeds which fall to the forest floor within the forest. The bank which defines the edge of the river is usually at a relatively steep angle compared to the forest floor. Seedlings then which germinate here and are anchored in the bank have their root structure fixed at an angle different from that of seedlings within the forest away from the river. The combination of the angle of rootage and the availability of light above and away from the forest edge defined by the bank causes the seedlings to grow outward--that is, at an angle--as well as upward. These seedlings will mature as trees with the bulk of their weight oriented at a shearing angle to their root structure: these trees are inherently unstable compared to any tree of equal diameter which has its trunk perpendicular to its roots. Such trees, then, because of the way they grow are the main candidates for becoming snags in the river. They become snags when the combination of erosive events and the leverage effects of their weight, height and angle of growth exceed the capacity of their root structure to support them.
One hardly need observe, however, that snags are not high on the agenda of interest or concern for most river users; in most cases it is quite the contrary. On the Elk River, for instance, from the point of view of recreational boaters [I am speaking here in terms of "no flow" conditions], snags are seen as impediments to passage because they obstruct the downstream route and in segments of faster water may be dangerous if a boat is overturned. Many of the people who treasure rivers for their scenic values see most snags as eyesores on the river--particularly after a flood or spate when larger snags have accumulated great mounds of sand, gravel, trash, and organic flotsam. Fisheries biologists, while concerned with the general features of fish habitat, are more oriented to the distribution of species, and the quantity and quality of fish, along with the general water quality considerations necessary to support the fishery; they seldom deal with the role of snags in fishery quality. Many other users of the river are interested in wildlife and wildflowers along the river, but usually do not pay much attention to snags as the critical link in a very diverse wildlife chain connecting the river bottom with the nearby bank. Snags and their attendant flora and fauna are just not the kinds of things people think about in looking at a river; in fact, most of the general books on river biology and stream ecology that I read do not mention snags at all.
In my trout fishing experience, however, all of these things come together in a complex whole system in snags. Snags are good places to find trout, particularly large brown trout [and sometimes large rainbows]. Why? Because the snag offers a variety of things the trout needs--in simple terms food and shelter, but the way a snag does this is somewhat complex. When the snag enters the water, it alters the flow regime of the stream at that point. Much like a wing dam or artificial deflector, the snag slows down a portion of the stream, even while it may cause flow at an adjacent point to increase. This flow differential combined with the physical shelter [overhanging limb, tree trunk, leaf mass, etc.] offered by the snag makes a good trout lie: the trout can, with minimal energy, maintain a feeding station in the relatively slow water flow of the snag but at the same time be located close to a lane of relatively faster water that will bring food items past the feeding station. Very often the phenomena I am describing here involves stream dimensions of just a few inches. In streams of low turbidity and relatively shallow water, snags also provide additional shelter by creating shade from the sun, and large trout [and other fish] can sometimes be observed in the middle of the day carefully maintaining position in the shadow of a log or limb.
In addition, the turbulence generated by the snag has other effects for the trout. The swirl or vortex of waters generated in the immediate area of limbs and twigs will sometimes de-stabilize emerging nymphs making them easier to be caught. Snags also typically generate one or more small eddies [some times only a few inches square] that cause floating terrestrials [non-aquatic insects] to slow down and be shunted into a very small area again making it possible for trout with minimal effort to feed around the snag. In the case of large diameter snags which lie perpendicular to the flow, the snag log may act as a kind of dam raising and slowing the water on the upstream side while creating a small waterfall on its downstream side. These cross snags create significant turbulence and mixing of the water and may alter the bottom form beneath and downstream of the snag. The snag and its turbulence also cause some limited heat loss in the water due to friction, thereby lowering temperature of the water. This effect when combined with the mechanical effects of turbulence can slightly increase water quality for the trout by increasing oxygen content although this effect is somewhat offset by the oxygen consumed by decay processes in other parts of the snag.
Snag phenomena have latitudinal [cross-stream] and longitudinal [downstream], and vertical features. Latitudinally, the snag may alter the form of the bottom so that the local bottom cross-section undulates: the bank, a small area of deeper water, the snag and its associated bottom debris, and the main channel and the opposite bank. The smaller channel of flow on the backside of the snag is important for several reasons: it keeps freshwater and food flowing into the snag in addition to that provided by the main current to the outside; it helps to flush the snag of debris, silt, and colloidal suspensions that would cause the snag to become impacted or lower the water quality; and this secondary current often provides an escape route for the fish exiting the snag area. Longitudinally, the snag separates the river bottom into corridors which physically isolate segments of the river from each other thus giving shelter to different types or sizes of fish. These corridors also segregate varieties of river usage: canoeists pass by in the more open main channel while fish find undisturbed shelter behind or under the snag. Vertically, the snag provides a means or route of access into and out of the water: for insects, snakes, amphibians as well as a means by which some types of chemical runoff [e.g., from bark, roots, vines, and leaves] are conveyed directly into the stream. In the vertical aspect the snag also provides a perch for mammals and birds using the stream or foraging in the rich environment of the snag.
A snag is much more than a tree or bush that has fallen into the water. When the snag enters the water a series of things begin to occur. Immediately the snag alters the flow of the stream and generates several forms of stream turbulence. This turbulence is important not only for its effects in allowing fish to capture food, but also for its eventual effects in allowing the snag itself to generate food. Turbulence is simply the manifestation of differences of flow. With respect to the load of silt carried in the streamflow, turbulence [as well as bends and meanders, waterfalls, riffles, etc.] causes an alteration in the hydrologic competency of the stream--increasing or decreasing stream competency depending upon whether the flow speeds up or slows down. Some parts of the snag, however, will always cause the streamflow locally to exhibit reduced competency. This means that in those areas of the snag, the silt, sand, gravel, [or in some instances rocks and boulders] entrained in the flow will lose their downstream momentum and fall out onto the bottom often in a very definite vertical and longitudinal gradient of type, size, and shape. Snags then are often silty and muddy--that is why they are unpleasant to wade through or cross, but this characteristic also reflects the work of the snag in cleansing a silty or muddy river; snags filter the river.
Snags accumulate silt and gravel: snags thus contribute locally to the building of stream bottom. This effect is important for several reasons. First the accumulated "silt and gravel" actually contains not only silt and gravel, but sand, saturated twigs and leaf fragments, and semi-dissolved solids from the decay of organic materials and some chemical precipitates: in short, the makings of the bottom of the food chain. Secondly the physical character of the mix of silt and gravel elements along with the shape of particular particles in the mix provide the physical substrate to protect primary organic processes as well as to provide shelter and points of attachment for the first phyto- and zoo-planktons to be generated in the snag. Very soon, algae, mosses, and grasses also begin to develop on this substrate and are the critical link in the production of oxygen within the stream. These plants also provide an element in the food chain, additional physical alterations in the chemistry and character of the water, and additional shelter to members of the food chain. As larger invertebrates occur, the coarser bottom particles along with the algae and grasses also provide shelter and attachments for these invertebrates as well. At this point the generation of a locally complex food chain is well underway, and the structure of snags can be seen to favor the development of specific faunal communities as is the case with pools or riffles.
Snags, however, are not only places where trout occur--because of the richness of the food supply--but also provide shelter to many lesser fishes. Many types of minnows, darters, dace, etc. occupy niches in and around snags. The snag shelters these small species of fish, but snags also concentrate them making the snag a desirable feeding station for larger fish such as brown trout or largemouth bass. Depending upon whether the snag is well-flushed by streamflow or is sufficiently large to generate a slack eddy or pool, other species such as bluegills, perch and crappie and bottom feeders such as carp and suckers may be present in the snag. Sometimes four or five species of fish can be taken from a single snag depending upon how it is fished, and the snag may--from side to side and top to bottom--exhibit distinct zonation in the types of fish present.
The snag also provides a feeding station for piscivorous mammals and birds: notably mink, otter, and kingfishers. The presence of birds and mammals around the snag brings us to another feature of snags: their role in an ecologic system that extends beyond the riverbanks. This larger snag ecology includes not only the riverine content of the snag but an extended community around the riverbank and ultimately includes the human community. I have said that snags are not just dead trees that fall into the water nor are snags merely the product of erosive events. Sometimes healthy or mature trees become snags. For instance, beavers may topple a large tree in order to make more of its bark accessible and such a tree may become a snag. In fact, beavers, otter, mink, muskrats, and to a lesser extent other burrowing mammals including rodents have a role in snag generation. Riverine mammals in particular are active agents in burrowing and thereby undermining or weakening the bankside security of mature trees. In addition, burrowing birds such as kingfishers may have a limited effect on snag generation.
In the river system there is an important interaction between snags and human activity. For instance, when farmers excessively clear their field edges, fence rows, and lanes or when industries and municipalities overpave in a water shed, stream courses can exhibit violent runoff during downpours or protracted rains. Violent runoff--even in flood control streams like the Elk--can generate many snags as excess water floods along banks and fields particularly where clearing has too greatly thinned the wooded cover along the stream. In such cases many trees may be uprooted and swept into or down the river; gigantic log/debris jams may occur; and massive bank caving can result. On the other side of this same phenomenon snags can mitigate some of these effects. Snags can, over the whole course of a river, slow down the flow of water and act as mechanical buffering devices to protect banks and other stream forms from excessive erosion. In some places where serious bank caving has resulted from violent flood erosion or land abuse, the resultant opening in the bank may serve as a trap for detached snags and the snag succession at that point can aid in healing what would otherwise remain a serious point of erosion on the river.
During floods or spates, snags can also protect a river in another way. During the severe flood conditions of the solstice storm of 1990 and the related flooding following in the spring of 1991, the spillway discharge from Tims Ford Dam was so great and contained within so narrow a river corridor that much of the stream bottom for about fifteen miles downstream was moved and overturned. This tillage upset the substrate of most of the main course of the river both cleansing the substrate but also breaking the food chain by sweeping and scouring of the riverbed. At some points in the riverbed in the late spring of 1991, the scouring was so complete that many snags appeared bright and smooth as if they had been sandblasted while the bottom gravel was bright and shiny as if it had been vacuumed. Often this scouring took place along corridors of the bottom in such a way that the fairly well defined edges between the scoured and non-scoured areas gave the appearance of a highly artificial line drawn on the bottom. This scouring and flooding largely broke the foodchain in the main course of the river. The river's recovery, however, was quickly nurtured from the protected areas under snags and from the snag channels on the back sides of islands. Were it not for the brood stock of plankton and invertebrates, algae and grasses, preserved in these snags, the recovery of the Elk would have been much slower. Such snags are an important nursery in the total system of the river.
On rivers such as the Elk which are used for power generation the availability of snags is an important element of shelter during generation. On the Elk, the bank-to-bank width averages under 100' and the "normal" streamflow is around 70-90 cubic feet per second. However, during power generation, flow increases to 3600-3800 cfs and the water deepens five to six feet filling to the flood banks. Although some feeding by fish continues to occur and some streambed insects continue to hatch, feeding conditions are difficult and a greater expenditure of energy is required by the fish to maintain stream position or to move into the main current. In addition, the bottom of the Elk for the most part is a mix of loose gravel, sand, and small rocks. Although some larger rocks occur, most average less than one cubic foot in volume and do not offer the downstream pocket protection to be found in other types of streams. Basically, the main channel of the river is an unprotected and relatively even gravel bed offering little in the way of shelter in any flow conditions. In these conditions, the presence of snags is important for offering resting places to fish outside of the strong generation currents.
In the spring of 1993, I observed a large cross-stream snag on the Elk River which exhibited another feature of snags. This snag consisted of four large trees wedged cross-stream against both banks of the stream and against a midstream bridge pillar. The snag zone also contained numerous logs, smaller trees, and large amounts of woodland and agricultural flotsam, along with quantities of trash mostly in the form of PET plastic bottles, plastic bags, and expanded foam bait cups, cooler material, and drink cups. The effect of this snag was to screen and concentrate virtually all of the floating debris moving down the river. This debris had accumulated in the snag in lateral bands five or more meters long and up to one half meter deep; the debris bands were complexly inter-layered with large and smaller organic elements tightly banded with expanded foam, shredded garbage bags, etc. The primary trees of this snag were prevented from downstream migration by the way in which they were wedged against the banks and the bridge, but the snag was not therefore static; in fact, it was very dynamic: the trees rose and fell with the undulations of the river and the protruding limbs reached both above and below the trunks and entrapped whatever was flowing down the river. The water beneath this snag was about one and a half meters deep and provided shelter for fish beneath and downstream of the snag. The surface water upstream of the snag and along the accumulated debris bands was a rich scum of pollen, seeds, and trapped terrestrial and aquatic insects. The hydraulic of this snag caused a stong current to plunge downward through and beneath the snag carrying food material to the fish below it.
It is obvious that snags must be seen as cyclic, dynamic forms in the streams. Although some snags last a long time, the trees eventually decay and are mechanically reduced thus adding their nutritive and chemical content to the natural content of the water. The turbulence and dis-entrainment that generates bottom structure under snags eventually lead to a transformation of the snag and to a transformation of the ecologic community related to it. Large snags have the capacity to make themselves even larger by trapping other trees and flotsam as well as by trapping large amounts of sediment. Eventually a snag may accumulate enough sediment that grasses will rise above the surface of the water, trapping even more flotsam and sediment until the snag may generate small temporary or semi-permanent islands which become populated by a whole new series of flora and fauna different from those which originally colonized the snag. If the snag island becomes sufficiently dry, trees--particularly sycamores, birches, and some dwarf elms--may appear and further alter the character of the snag island. The presence of tree roots will be critically important in protecting the snag island against reduction in subsequent floods. Such trees do not have to be very large to be effective in protecting the snag island from erosion.
In a few cases, the snag may accumulate enough sediment so that its island connects itself to the riverbank on the inside of the snag. When this happens a long downstream bar may emerge below the island. Sometimes the joining of a snag island to its nearest bank can result in the narrowing of the streamflow at that point. This narrowing can cause the current to increase its speed thus deepening the channel between the snag and the opposite bank or it can deflect the stream causing it to bend away from the snag area and thus erode the bank on the side opposite the snag. These effects can be observed on many streams including the Elk, but they are seldom seen as the result of trees falling into the river or the complex series of very dynamic consequences that follow from these trees.
I have argued the case of snags at some length because they are river forms that are almost routinely dismissed or ignored and would certainly receive very low marks from most people--including some professionals--in evaluating a river reach. Certainly I think most people who use rivers do not see snags as key items in complex if small ecosystems nor do they recognize the double cycle exhibited by the snag system within the river itself and between the snag form and the related flora and fauna of the bankside ecosystem. Snags, however, are both as natural to river systems as pools and riffles and they are critical in maintaining river quality both locally and over the whole course of the river. Snags are, on a small scale, what rivers are on a large scale: that is, they are complex wholes that cannot be understood piecemeal--no matter how accurate and detailed our enumeration of components is. We have not understood a snag when we say it is an obstruction or a tree in the water or a mud trap. It is of course all of these things, but it is also the key element in a large, complexly interrelated ecologic--and recreational--community that reaches beyond the riverbanks.