Water in wheels . Cause ?

Lucky Luke

After a very wet ride yesterday, my back wheel had taken on a considerable amount of water. This is not the first time this has happened. I took off the tyre and it was taken aback over the amount of water in there half a pint at least I would estimate. I've had to remove the tyre inner tube and even the rim tape to dry everything out. There's water within the rim (Mavic Aksium) so I've stood the wheel up so it drains from the valve hole.

Any ideas how this could be happening?