Reading a valley: Part 1

I had a new paper come out this month, entitled “Numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations.” The paper and supplemental movies are up now at Journal of Geophysical Research – Earth Surface. This work developed from the last chapter of my thesis with my adviser and co-author Mike Lamb.  The question: how do we read geologic history from the shape of a river valley?

First, a little background. Most of us have some experience with rivers, which are hubs of human life worldwide. Close to home, the St. Anthony Falls on the Mississippi River were the nucleation point for the city of Minneapolis. The river has been central to the regional economy as an engine for flour mills and electricity, a highway for commercial shipping, and even the source of most of the world’s buttons. In nature, rivers also serve as hubs of activity for evolving landscapes. Rivers are conduits for the movement of sediment, water, and nutrients across continents to the ocean. In the life of a landscape, however, rivers are more than conveyor belts: rivers are also great integrators. Tectonics, climate, vegetation, and human disturbance all influence rivers, and rivers in turn shape the landscape around them. And lucky for us, the Earth’s surface and sediments hold a “fossil” record a river’s work over geologic time.

One of the most common remnants of a river’s activity is a river terrace. As its name implies, a river terrace is a flat area–in fact, it usually looks like a valley bottom. But there’s a twist: river terraces are former valley bottoms, abandoned so far above the active river that they are rarely (if ever) inundated in floods. Terraces often occur in a series of steps, and in an architectural mixed metaphor, a set of terrace steps that descend toward the modern river is called a “flight.” Here’s a spectacular flight of terraces from the Wind River valley, Wyoming from Hancock et al. [1999]: 

Terraces along the Wind River, Wyoming (Hancock et al. [1999], their Figure 4).
For another example, take a look at this classic Ansel Adams portrait:

“The Tetons and the Snake River” by Ansel Adams, 1942.

Between the Snake River in the foreground and the jagged Teton Mountains in the background, you’ll note several broad river terraces.

River terraces are usually subtle in profile, and hide in plain sight in river valleys across the globe. In some landscapes, terraces can be dated to hundreds of thousands of years old. These river terraces are some of the best available records of river response to environmental change. In fact, the most common explanation for why rivers abandon terraces in the first place is that climate change disturbs the balance of water and sediment delivered to the river, causing it to cut down further into its own valley. Thus, terraces hold key clues to climate in the past, and are vital to placing modern landscape change in context.

There’s a snag, however: rivers may make terraces of their own volition, without any changes in climate. Consider, for example, an experiment Les Hasbargen made here at St. Anthony Falls Lab in the late 1990s. In the experiment, a ~0.5 square meter patch of sediment served as the initial landscape, and was subjected to steady “rainfall” in the form of misted water droplets and steady “uplift” to build topography. A vivid landscape in microcosm burst into bloom, with famously shifting mountain ridgelines and yes, terraces. This sandbox-scale experiment showed that under simulated conditions with steady climate and tectonics, landscapes can develop features like river terraces that would otherwise seem to require climate change. Resolving terrace origin is difficult  using field observations alone, in part because river terraces are thought to form over millennial timescales.

So how can we reconstruct a valley’s history after arriving so late in the game? Stay tuned for Part 2.