Morphological Evolution of a Small Rural Stream over 143 years in Southwestern Ontario, Canada

Abstract:

Parkhill Creek is a small rural headwater stream that discharges into the larger Ausable River, eventually draining into Lake Huron in Southwestern Ontario, Canada. In this study, the evolution of Parkhill creek was characterized by identifying the key contributions to morphologic change in the watershed. Data were collected from georeferenced maps and aerial photographs from the years: 1887, 1939, 1954, 1970, 2002, 2006, 2015, and 2016. Areal extents of fields and natural land, as well as stream length and straightness, were acquired from aerial photographs. The main factors that impact the shape of the stream were identified as; dredging of ditches and other agricultural drainage installations; the installation of a dam and reservoir in 1968; and the clearing of riparian; forested; and other natural landscapes for agriculture and related land uses. The amount of land cleared for agriculture between 1954 and 2015 was 1000 ha and the mean field size increased from 5.5 to 13.7 ha. Larger fields and the clearing of more land resulted in an increased sediment supply to the creek. Channel dredging and straightening practices were widespread in the watershed as early as the 1950’s resulting in greater sediment transport rates and an increased runoff response rate to precipitation events. The dam was installed in 1968 near the town of Parkhill to help resolve water quantity issues in the summer months since the channel would often run dry. It was also used to control flow in the mid to late winter when flooding was common. The dam raised the base level of the stream from 185.41 to 192.71 meters above sea level (an increase of 7.3 meters), resulting in immediate disequilibrium conditions both upstream and downstream of the reservoir. Aggradation of sediment upstream of the reservoir reached up to a minimum of 1 km two years post installation. Erosion has clearly modified the shape of the stream downstream of the dam. This has resulted in the draining of downstream ponds causing previously permanent ponds to be ephemeral and changing the morphological pattern of the meandering stream planform in the floodplain. Understanding the history of a stream is integral to studying potential impacts of climate change, land management scenarios, and the sustainability of current land management practices.