Thank you EJ, Gamera, and T.B.
Originally I wanted to show river flow direction on each of the bridge footings, etc,. However, the paper mache task itself was enough for me, so I dropped the idea. Besides, the river is tidal, so it has a slack tide, with no flow (that is my excuse).
Also, the tide is normally quite slow and hardly noticeable when examining the water flow around the footings. The only time it becomes really noticeable is during flood on an ebb tide.
The river colour is normally monotone brown-green like lentil-pea soup colour. This is mainly due to periodic rains. When there is a drought, the water clears up a bit, and becomes more of a teal colour (but still showing deep along the shallows).
However, I plan to make the water of varying shades of dark to light, indicating river depth. This is purely for educational reasons. It will show why the bridge-span was not just placed in the centre of the river. Below shows what I have in mind, of course the coloured strips will be blended into each other (hopefully).
Thank you T.B. for those helpful tips and reminders. They brought images and ideas to mind, and will try and incorporate some of them into the painting part. My primary goal is to somehow mix the shades from dark to light. I am going to use acrylic paints, so I am going to have less time to get it right, especially under the bridge.