This is the source of the River Severn. As you can see it is wet and boggy, unlike the source of the River Thames.
The River Severn starts just down from the top of Plynlimon in the middle of Wales. The surrounding land is all relatively flat hill top, so where did all the water come from?
At the coast you can watch the tide come in and see water flow into the mouth of the river (quite dramatically in the case of the Severn), but it doesn’t go in very far.
Water can’t flow uphill, so as soon as the land starts to slope the sea can no longer influence the river. All the flow moves downhill.
Water at the source of the river has come from the sea, but via evaporation, cloud formation and rain.
With rain on more than a third of the days in every month and a total of 2650 mm for the year (more than a tropical rainforest), there’s plenty to keep the bogs of Plynlimon topped up with water. This gradually trickles downhill gathering in a stream which forms the River Severn.