River valleys come with a wide range of shapes, from narrow canyons to wide plains. We know very little about what controls their width. To first order, wide valleys occur in big rivers. Indeed, compilations show a relationship between water discharge and valley width. They also show that valleys narrow as valley walls get harder to erode. However, widths scatter over multiple orders of magnitude for the same water discharge and valley-wall lithology. Something more controls the shape of valleys.

To investigate these additional controls, we turned to paired river terraces. Paired river terraces preserve the geometry of past valley shapes at a single point along the river. Moreover, many terrace sequences can be linked to cycles of wet and dry climate. In that case, all terrace levels preserve valleys that were formed under similar climatic and lithologic conditions.

We compiled valley widths from 12 globally distributed – and climatically formed – terrace sequences. For all sequences, we find a very clear relationship between the height and width of the valley (The height refers to the height of the entire valley wall, not the height of individual terraces). This finding raises the following question: Does a process related to valley height impact the width of valleys?

Based on our observation, we propose a new model for valley formation. Rivers widen valleys by lateral erosion of the valley walls. The eroded sediment has to be removed before erosion can continue. At the same time, valley hillslopes are eroding and deliver sediment to streams. A linear relationship between valley width and valley height can be explained only when sediment supply from hillslopes and sediment removal by streams are in balance. Hence, sediment supply from hillslopes may limit valley widths.