Land use, sediment supply and channel response of southwest Ohio watersheds

The goal of this dissertation is to evaluate the historic impacts on the stream systems in southwest Ohio, US. This region experienced widespread deforestation for development of agricultural land since the early 19th century, which has had a significant impact on watershed hydrologic processes. Understanding the historic impacts is critical in stream management and restoration where stream erosion is common in this region.

The first part of the dissertation examines the interaction between sediment supply and channel insta¬bility and its implications for stream restoration. Channel instability was found to be largely a consequence of reduced sediment supply and increased transport capacity over past several decades. Because sediment supply and transport capacity are governed by watershed-scale conditions, reach-scale restoration neither improves water quality nor prevents channel instability.

The second part of the dissertation quantifies the past, present and future impacts of land use, channel incision and climate on hydrologic regime. Results show that flood peak discharge was more than three times higher today than in 1800 and will be five times higher in 2050 than in 1800. Stream power will also increase by a factor of two between 2000 and 2050. This will substantially increase transport capacity and exacerbate the widespread stream erosion problem.

The third project investigates the cumulative impacts of multiple impoundments on hydrologic regime and sediment flux. This study demonstrated that impoundments reduce 37% of the flood peak discharge and 30% of the sediment yield. These ubiquitous impoundments are sediment sinks. The reduction of sediment yield along with the improvement of soil conservation practices can significantly reduce the sediment delivery to the streams. This reduction is one the root causes of the stream instability problem in southwest Ohio.

Lastly, the potential for using terrestrial gastropod shells to date historic stream deposits was evaluated. The shells of living gastropods were radiocarbon dated and it was found that the Mesodon species yields erroneous ages as a result of limestone ingestion. However, amino acid racemization (AAR) ratios of fossil shells, from strata of known age, demonstrated that the AAR method is capable of differentiating the historic alluvium from older alluvium.