The rope you need is in plain view, but it's out of immediate reach.

Something's holding it just above your head.

Have you looked at the pond at the end of the dry river bed?

It's on the map screen immediately north of the map screen which contains the south shore.

The hanging corpse is holding a coil of rope.

But you can't reach it unless either you get up there or...

He gets down.

Somehow you have to lower the hanging corpse from its perch.

Have you noticed what the cord suspending the corpse is tied to?

A log.

To lower the body, you have to raise the log.

The log is too heavy to lift.

What would raise the log, though?

The corpse is hanging over the remnants of a dried-out pond.

Logs float.

If the pond was full, the log would float.

In order for the pond to be full, the water would have to come along the dry river bed.

What's holding back the water?

A dam is hindering the water at the river fork (on the same map screen as the pond).

Somehow you need to unlock the dam.

How can I unlock the dam?