Hello ShapeshiftI don't have any suggestions that involve calculus, but I do have some that use similar triangles. (And I'm assuming that you can cross the river, and that 'Environment' includes things like trees, buildings, and other random bits of stuff.)

1 Find a 'random bit of stuff' about, say, 2 m in length; e.g. a piece of wood, a willing friend, etc.

2 Measure its length carefully with the 10 cm ruler.

3 Find a tree, tall building, etc near the bank of the river on the side which you currently are on.

4 Lie on the ground so that your eye is just above ground level. Mark on the ground the position of your eye.

5 Position the 'random bit of stuff' in such a way that its height appears to be the same height as the tree (building, etc.)

6 Use the ruler to measure the distance between the position of your eye and the foot of the 'bit of stuff'.

7 Use the bit of stuff to measure the distance between the position of your eye and the foot of the tree.

8 Use similar triangles to calculate the height of the tree.

9 Cross the river and position yourself on the bank directly opposite the tree.

10 Repeat steps 4 to 6.

11 Knowing the distance of your eye from the 'bit of stuff', the height of the tree and the height of the 'bit of stuff', use similar triangles to calculate the width of the river.

Grandad