How much of the precipitation that falls to the earth flows into rivers?

When a dam is built in a hot, dry area, creating a lake or reservoir aboved the dam, how does the evaporation from that reservoir affect the environment ?

Why does physical weathering speed up the processes of chemical weathering? a) physical weathering requires abundant water, as does chemical weathering. b) physical weathering produces fresh surfaces for chemical weathering to attack. c) chemical weathering can occur only on small surfaces, not large ones. d) chemical weathering requires salt, which is provided by salt wedging.

At which point on the diagram do convection currents occur to drive plate movement?