The foregoing statements make it clear that a stream may be eroding in one part of its valley while it is depositing in another, and that erosion may alternate with deposition in the same place, on account of fluctuations in volume, and, therefore, in velocity of the stream. It will be seen in the sequel that erosion and deposition may be taking place at the same time in the same part of the valley. The activities of a river are so nicely balanced that slight disturbance at one point causes disturbance at all points below. _The deposits._ =Types.=—Turning from the principles which underlie river deposition to the deposits themselves, they are found to occur in various situations. Running water usually descends from steeper slopes above to gentler slopes below, and ends at the sea, or in a lake or inland basin. Wherever there is a sudden decrease in its gradient, as at the base of a hill, ridge, or mountain, running water is likely to leave a large part of its load, building an _alluvial fan_ or _cone_ (Figs. 67, 68, and Pl. VI). Even where there is no sudden decrease in the gradient of a stream, there is likely to be a gradual one, and in spite of the fact that the increased volume of a stream in its lower course tends to overcome the effect of diminished gradient on velocity, deposition is likely to take place as the gradient is reduced. Deposits occasioned by the gradual reduction of a stream’s velocity often have great extent in the direction of a stream’s flow. They cover the flood plains of streams, making them _alluvial plains_ (Fig. 73). When a stream reaches the sea or a lake its current is destroyed and its load dropped, unless taken in charge by the waves and currents of the standing water. Sediment accumulated in quantity at the debouchures of streams gives rise to _deltas_ (Figs. 169, 187). Alluvial cones and fans, alluvial plains, and deltas, are the principal types of river deposits. Apart from these well-defined types there are _bars_ in the channels of depositing streams, and much ill-defined alluvium which does not allow of ready classification. =Alluvial fans and cones.=—The only distinction between the alluvial fan and the alluvial cone is one of slope, the cones (they are but half-cones at best) being steeper than the fans. Alluvial fans and cones have their most striking development where temporary torrents, occasioned by showers or the rapid melting of snow, issue from mountain ravines. Such streams usually carry heavy loads of detritus, the coarser part of which is likely to be deposited at the base of the mountain slope. Cones and fans built by such streams have a periodic rather than a steady growth.