RIVERS AS AGENTS OF LANDSCAPE EVOLUTION

Rivers as agents of landscape evolution Every landform at the Earth's surface reflects a particular accommodation between properties of the underlying geologic materials, the type of processes affecting those materials, and the amount of time the processes have been operating. Because landforms are the building blocks of regional landscapes, the character of the local surroundings is ultimately controlled by those factors of geology, process, and timea conclusion reached in the late 19th century by the noted American geologist and geographer William Morris Davis. In some regions, severe climatic controls cause a particular process agent to become preeminent. Deserts, for example, are often subjected to severe wind action, and the resulting landscape consists of landforms that reflect the dominance of erosional or depositional processes accomplished by the wind. Other landscapes may be related to processes operating beneath the surface. Regions such as Japan or the Cascade Range in the northwestern part of the United States clearly have major topographic components that were produced by repeated volcanic activity. Nevertheless, rivers are by far the most important agents in molding landscapes because their ubiquity ensures that no region of the Earth can be totally devoid of landforms developed by fluvial processes. Rivers are much more than sluiceways that simply transport water and sediment. They also change a nondescript geologic setting into distinct topographic forms. This happens primarily because movement of sediment-laden water is capable of pronounced erosion, and when transporting energy decreases, landforms are created by the deposition of fluvial sediment. Some fluvial features are entirely erosional, and the form is clearly unrelated to the transportation and deposition of sediment. Other features may be entirely depositional. In these cases, topography is constructed of sediment that buries some underlying surface that existed prior to the introduction of the covering sediment. Realistically, many fluvial features result from some combination of both erosion and deposition, and the pure situations probably represent end members of a continuum of fluvial forms. Valleys and canyons River valleys constitute a major portion of the natural surroundings. In rare cases, spectacular valleys are created by tectonic activity. The Jordan River and the Dead Sea, for example, occupy a valley that developed as a fault-bounded trough known as a rift valley. The distinct property of these and other tectonically controlled valleys is that the low topographic zone (valley) existed before the river. Notwithstanding tectonic exceptions, the overwhelming majority of valleys, including canyons and gorges, share a common genetic bond in that their characteristics are the result of river erosioni.e., rivers create the valleys in which they flow. In most cases, erosion was accomplished by the same river that occupies the valley bottom, although sometimes rivers are diverted from one valley into another by a process known as stream piracy, or stream capture. Piracy of a large river into another valley often creates a situation where the original expansive valley is later occupied by a river that is too small to have created such a large valley. The opposite case also may occur. The implication here is that valley size is directly related to river size, an observation that generally holds true. Exceptions to this rule arise because of capture events during the evolution of a valley and because valley morphology is strongly influenced by variations in the bedrock into which the valley is carved. A genuine bedrock valley is usually covered by valley-fill deposits that obscure the actual configuration of the valley floor. Therefore, little is known about valley morphology unless drill holes or geophysical techniques are employed to document the buried bedrock-alluvium contact. Where information is available, it suggests that the deepest part of most valleys is not directly beneath the river. Commonly, the influx of load at a tributary junction forces the river to the opposite side of the valley, a phenomenon demonstrated clearly in the upper Mississippi River Valley between St. Louis, Mo., and St. Paul, Minn. Where a valley is devoid of thick deposits and is completely occupied by a river, the bedrock valley floor often develops an asymmetrical configuration such that the deepest part of the valley occurs on the inside of bends. This general rule is not inviolate because the position of incision depends on the amount of load entrained by the river. When sediment load is totally entrained and velocity is high, entrenchment will most likely occur on the inside of the bend. If deposition occurs or sediment cannot be entrained, however, incision will normally be on the outside of the bend. In straight reaches the deepest part of the valley floor is normally associated with an inner channel cut into bedrock. Its position is determined by where the river was at the time that it flowed at the level of the valley floor. Inner channels form as the culmination of a progressive change in erosional features during the initial phase of incision. Scour features gradually coalesce until a distinct channel appears that is able to contain the entire river flow. Inner channels are rarely seen except when exposed during excavation associated with dam construction. Where observed, such channels commonly have a narrow, deep gorgelike shape. For example, at the site of the Prineville Dam in the state of Oregon, the inner channel averages 21 metres wide and as much as 18 metres deep.