Principles involved.—Since deposition results from the failure of transportation, the factors which control transportation also influence deposition. Transportation by streams is determined largely by velocity, and the most important factors influencing velocity are slope, volume, and load ([p. 115]). Of these the first two are usually of greater importance than the third.
A stream is said to be loaded when it has all the sediment it can carry; it is loaded with fine material when it has all the fine material it can carry, and with coarse material when it has all the coarse it can transport. A stream loaded with coarse material flows more swiftly than one loaded with fine, for a larger percentage of a stream’s energy can be utilized in carrying fine material than coarse, and hence a larger percentage of the energy of a stream which carries a load of the latter will express itself in velocity.
Deposition takes place whenever a stream finds itself with more load than it can carry, and is an expression of the stream’s refusal to remain overloaded. A stream may become overloaded in various ways. It might at first seem unnecessary to inquire whether a stream may be overloaded at its source, but the question is not necessarily to be answered in the negative. The source of a stream is not always a definite point. In a general way it may be said that the source of the normal stream is at that point in its valley where the bottom is as low as the ground-water level of the region. But since the ground-water level is not constant ([p. 71]) the source of a stream is likely to be farther up its valley in a wet season than in a dry one ([p. 72]). After a heavy shower, the run-off descends to the axis of the valley from the slopes on all sides, and temporarily the stream begins above the point which marks even its wet-season source. If under such circumstances the slopes about the head of the valley are notably steeper than the slope of the valley itself, as they frequently are, the water flowing down them may gather an amount of material which it cannot carry after it reaches the bottom of the valley. This may be the case at, or even above, the point which marks the source of the permanent stream. It is, therefore, possible for a stream to be overloaded at its source, if we take the source to be the point whence the water permanently flows. Deposition may, therefore, be taking place in a valley at the head of its permanent stream, or temporarily even in the valley above it.
Streams issuing from glaciers sometimes have more load than they can carry after they escape from the ice. If the stream be regarded as beginning at the point where it issues from beneath the ice it may be overloaded at its source.[69]
Under certain circumstances, a stream may overload itself. Thus if a stream loaded with coarse detritus reaches a portion of its valley where fine material is accessible in abundance, some of the velocity which is helping to carry the coarse may be used in picking up and carrying the fine. This reduces the velocity, and since the stream already had all the coarse material it could carry, reduction of velocity must result in deposition. It follows that when a stream fully loaded with coarse material picks up fine, it becomes overloaded, so far as the coarse material is concerned.
Again, tributaries may overload their mains. While tributaries are usually smaller than their mains, they frequently have higher gradients, and the smaller stream of higher gradient may bring to the larger stream of lower gradient more material than the latter can carry away. Thus deposition may take place at the point of junction of tributaries with their mains. This may go so far as to pond the latter enough to cause its expansion into a river-lake. Lake Pepin, in the Mississippi River at the mouth of the Chippewa (in Wis.), is an example.
Streams may become overloaded by losing velocity or volume, or both. Decrease in velocity is brought about either by decrease in declivity or in volume. In general, streams have lower gradients and greater volumes in their lower courses than in their upper, and these two elements affect velocity in different ways. If the increase in volume be not enough to counterbalance the decrease in declivity, as is often the case, a stream which is loaded in its upper course will deposit in its lower. The decrease of velocity at the debouchure of a stream almost always leads to deposition.
Decrease in velocity as the result of decrease in volume is less common. When decrease in volume occurs, it may be the result of (1) evaporation, (2) the absorption of water into the bed of the stream, or (3) branching—the giving off of distributaries. While evaporation is going on everywhere, the diminution of a stream by this means is usually more than balanced by the increase from tributaries, rainfall, and springs; but in arid regions a very different condition of things sometimes exists. If mountains in an arid region be capped with snow, its melting supplies the streams during the melting season. As the streams flow out from the mountains through dry regions, they receive little or no increment from rainfall, tributaries, or springs, and evaporation reduces the volume of water, or even dissipates it altogether. Absorption of water into the bed of the stream often accompanies evaporation. Reduction of volume by evaporation and by absorption is especially common in arid regions. Wherever loaded streams are reduced in volume, whether by evaporation or absorption, deposition takes place.
The third way by which velocity is decreased as the result of decreasing volume is illustrated at the debouchures of many streams. Near the Gulf, for example, the Mississippi branches repeatedly (see [Fig. 190]). The same phenomena are often seen where one stream joins another ([Fig. 169]). Individually the distributaries are much smaller than the main stream before they separated from it, and because they are smaller their combined surfaces are greater, and the amount of energy consumed in the friction of flow is increased. The velocity of the water and its carrying power are, therefore, reduced. Thus the branching of streams gives rise to deposition, and where deposition takes place the gradient of the stream is reduced, and this occasions still further deposition. The sediment which fills up the channel and checks the flow finally compels the stream, or some part of it, to transgress its banks. Deposition, therefore, favors the development of distributaries, and the development of distributaries in turn favors deposition.
