River, Not a Falls

I'm sure you have seen things like this.

Credit River from the Forks Bridge

Or you might have seen it thus.

Same Bridge, Different Angle

Any number of times I have seen painters paint this per the following sketch (exaggerated to explain the point).

Stripe or Falls? vs River

These images somehow seem to work for the painter.  I have seen quite a few like this, even hung for display and sale.  But it not a vertical stripe or waterfall.

Harold would tell you that you are not painting what you see - you are not looking, rather painting what you THINK is there.

Here is part of the mystery linked to seeing.

Water in Mug

The water is level with the ground.  In this case the mug is also "level".  That would mean that the water is not moving.

Water in Tilted Mug

The water has still found its level even though the mug is tilted.  The mug is like the land that contains a river.  You can see that the water wants to run - be a river.  BUT it is still level.  It will stay that way unless the land has a discontinuity in it producing a jump down for the water.

So this "level" water is key to making the river look like it is meandering along its banks, not dropping in a precipice.  Look at the subject, it will tell you how to paint it.  Then you as a painter can make decisions regarding eye line, perspective, how to position the river in the frame and so forth.