What characteristics of rivers at their source gives them so much power to shape the land?

Hydraulic action.

Explanation:

The hydraulic action s the capability of the river to erode which when occurs into motion against the surface of the solid rocks, ad produces a mechanical weathering or physical disintegration of rocks and thereby to transport the rock particles. Abrasion, attrition and corrosion and siltation and the downcutting of the ricks are thus an essential feature of this process. And hence with the rapid flow of the water, the cracks of the rocks develop and they become wide-open and thus acts as a wave that stricks the rocks in the ocean.