Which graph shows how changes in stream discharge usually affect stream velocity?

Graph A.

Explanation:

Stream discharge tells us how much (what volume) of water flows in an unit of time. It is usually calculated by multiplying velocity of water by a cross-sectional area (surface) through which water flows. So, flow (discharge) equals to velocity times area, or written in formula: Q = V x A. From here, we can see that Q (stream discharge) and V (velocity) are directly proportional; if one of them enhances so does the other, and vice versa. The only graph that corresponds to this relation is the first one.  

Graph B shows that discharge grows if the velocity remains constant. Graph C shows that first discharge grows if velocity drops, but later they both grow, but not linearly. Graph D shows that discharge grows if the velocity drops.

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