River Terraces

A river terrace is, at its simplest, an old valley floor or an old floodplain that has been left standing above the present-day river. Think of it as a step or a shelf cut into the sides of a valley, marking a level where the river once flowed but no longer reaches, except perhaps during the most extreme floods. The very existence of a terrace tells you that the river has changed its behaviour over time — it has stopped building up that particular surface and has instead started cutting down through it.

Terraces are not all built the same way. Some are bedrock surfaces, meaning the old valley floor is bare rock with no layer of sediment sitting on top of it. Others are alluvial terraces, where the old floodplain is still covered by the stream deposits — the sands, silts and gravels — that the river itself laid down when it was actively building that level. The distinction matters because it tells you what the river was doing at that stage of its history: depositing material, or simply carving into solid rock.

The key idea to grasp is that river terraces are products of erosion, not deposition. This sounds odd at first, because they are often made of stream deposits, but the terrace itself is created when the river turns from a phase of deposition to a phase of vertical erosion. The river first builds up a floodplain by depositing sediment. Then, for some reason — a drop in base level, a change in climate, or a tectonic uplift — the river begins to cut vertically downward into that very floodplain it created. The old floodplain surface is left stranded above the new, lower river level, and that abandoned surface is the terrace. The deposits on top of it are simply the remains of the earlier depositional phase; the terrace form itself is the result of the later erosional phase.

Because a river can go through several such cycles of deposition and down-cutting, you can find a number of terraces at different heights along the same valley. Each terrace level represents a former river bed or floodplain level from an earlier stage of the river's history. The higher terraces are older; the lower ones are younger. Reading these steps is like reading a staircase of the river's past — each one records a time when the river was flowing at that elevation before it cut down to the next level. …