A clearer and, or more detailed question is needed.
maybe a valley?
Runoff
It could be called runoff, feeder stream or source.
Yes, it is correct.
Waste water is were unused water flows down into the sea.
Water always flows down a river, so upstream is the opposite direction from the direction in which the water flows.
Water flows down divided.
The answer is... A river only flows down hill
If the water tank is above the solar collecting panels, then the hot water rises into the tank by itself and colder water flows down into the panels.
it speeds up
The general direction a river flows is down. The water responds to the pull of gravity, and it is pulled down - whichever way that happens to be (north, south, east, west, or whatever). Water flows downhill. Simple and easy. Nothing tricky about it.
water flows down hill because of gravity
A solar heater is called a self pumping system because cold water flows down the panels by itself when hot water rises into the tank.
The river usually flows downwards due to gravity, following the path of least resistance towards lower elevations or towards the ocean.