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.