Earth Science?

Question

Give a few reasons why the velocity of a stream might change over the course of a year. This change in velocity could account for the picking up and deposition of sediments along the channel of a river, not just as the river enters the ocean.

Answer 1

Variation in rainfall (input), particularly flooding events, is the main reason stream velocity changes in the short term.

Here's an excerpt from a good summary:
http://www.columbia.edu/~vjd1/streams_basic.htm

Flood Erosion and Deposition: As flood waters rise, the slope of the stream as it flows to its base level (e.g., the ocean or a lake) increases. Also, as stream depth increases, the hydraulic radius increases thereby making the stream more free flowing. Both of these factors lead to an increase in stream velocity. The increased velocity and the increased cross-sectional area mean that discharge increases. As discharge and velocity increase so do the stream's competence and capacity. In the rising stages of a flood much sediment is dumped into streams by overland flow and gully wash. This can result in some aggradation or building up of sediments on the stream bed. However, after the flood peaks less sediment is carried and a great deal of bed scouring (erosion) occurs. As the flood subsides and competence and capacity decline sediments are deposited and the stream bed aggrades again. Even though the stream bed may return to somewhat like its pre-flood state, huge quantities of sediments have been transported downstream. Much fine sediment has probably been deposited on the flood plain.


Basically:
Anything that changes the streams gradient will affect the velocity. Lowering the base level (the level of whatever body of water the stream discharges into) or uplift of the area the stream flows through for example will increase the velocity.

Answer 2

velocity is on direction and time, so after time the direction changes so the pebbles in the river would shift because of the mass of the pebbles would be a gravitational force to the earth