You don’t often hear “stratigraphy” and “stream restoration” in the same sentence, but Professor John Holbrook thinks that could change. Holbrook, a professor of geology at the University of Texas, Arlington with a special interest in fluvial sedimentology, is currently researching two practical applications of stratigraphy to stream restoration:

1. Using a stratigraphic map of the valley alluvium to provide critical insights into where to best place restored channel and floodplain elements; and
2. Using the stratigraphy to provide a much more complete picture of the channel’s behavior under various discharge scenarios than historical records can provide.

Mapping the alluvium
The reason for paying attention to the surficial alluvium, says Holbrook, is that it forms the foundation for the biological and physical processes in the channel-floodplain system, primarily by determining local moisture retention, infiltration and flow patterns. Once you map out the fundamental stratigraphic units of the valley alluvium – the channel loops, point bars, flood deposits, mud caps on the point bars, and splays – you'll know which places are likely to hold water after floods and where water is going to infiltrate down into the floodplain aquifer and work its way back to the river.

Holbrook and his students are using this concept to assist in restoration work at the Overton Bottoms North Unit of the Big Muddy National Fish and Wildlife Refuge on the Missouri River, and have published their work in a USGS Scientific Investigations Report.

One puzzle they've been able to solve with their stratigraphic maps is why the forest has been able to re-establish itself in some areas of the Refuge and not in others. These forests are primarily cottonwoods and willows, and the maps showed that the successful re-growth areas coincided with old channel deposits. These channel fills, explains Holbrook, are normally at topographic lows where water collects, and are fine-grained, low-permeability strata that can retain enough of that water to sustain cottonwood and willow regeneration.

“What we’re saying is that the best way to go about regenerating self-sustaining wetlands is put them where they used to be – around the old channel loops and courses,” says Holbrook, because these areas are more likely to support the standing water necessary for a thriving wetland.

Channel Design
Another application of stratigraphy to stream restoration being explored by Holbrook is its ability to provide guidance on designing restored channels. The first thing you want to come up with, says Holbrook, “is an accurate baseline as to what the channel wants to look like. Hopefully, you can then develop the restoration design to be as close to that as possible.”

Why look to the stratigraphy for answers? “It’s really difficult, using sheer input models, to understand exactly why rivers braid or meander or to predict how they will change form,” says Holbrook. “Even if that weren’t the case, rivers have often been so altered from their original states, in terms of sediment and water inputs, that the ‘historical form’ we know about may not be an accurate reflection of what the river will actually do.”

Having mapped the valley's stratigraphy, the next step he and his researchers are working on now at Overton Bottoms is using paleohydrology techniques to reconstruct what the sediment and water inputs to the system were at those times. In effect, they will try to develop a menu of past sets of input conditions and what the Missouri River’s response was to them. This will give the Refuge an idea of how the river may respond to the combinations of sediment and water they expect to see going forward.