Influence of Historical Land-Use Change on Contemporary Channel Processes, Form, and Restoration

Abstract

Big Harris Creek, North Carolina, possesses a geomorphic history similar to many drainages in the southern Appalachian piedmont, and was used herein as a representative example of the influence of European settlement on contemporary channel form and processes. The integrated use of historical, dendrogeomorphic, stratigraphic, and cartographic data shows that the conversion of land-cover from a mix of natural conditions and small farms to commercial cotton production in the late 1800s and early 1900s led to significant upland soil erosion, gully formation, and the deposition of legacy sediments on the valley floor. Aggradation was followed by catchment-wide channel incision in the mid-1900s in response to reforestation and the implementation of soil conservation measures. Collectively, the responses form an aggradational-degradational episode (ADE) that produced the geomorphic framework for the contemporary processes operating along the drainage network. Defined, characterized, and mapped process zones (stream reaches of similar form and process) show that the type, intensity, and evolutionary sequence of geomorphic responses varied within the catchment as a function of the position along the drainage network, the erosional resistance of the underlying bedrock, and the valley characteristics (particularly width). Understanding the spatially variable influences of the ADE on contemporary, reach-scale geomorphic processes provides valuable insights for restoration as it helps inform practitioners of the sensitivity and ways in which the reach is likely to respond to future disturbances, the potential impacts of processes on proposed manipulations intended to achieve the project’s restoration goals, and the potential risk(s) involved with channel reconstruction. The latter is strongly controlled by geotechnical differences between erosionally resistant precolonial deposits and easily eroded legacy sediments that locally form the channel banks following the ADE.

Application of Chemostratigraphic Methods to Floodplain Alluvial Deposits within the Big Harris Creek Basin, North Carolina

“…The Big Harris Creek Basin, a tributary to the First Broad River, is the site of one of the largest stream restoration projects in North Carolina, a project that encompasses the upper 9.6 km 2 of the basin. Detailed analyses, which included the interpretation of historical, stratigraphic, dendrochronologic, and cartographic data, demonstrated that the geomorphic history of the basin was similar to many other catchments within the southern Appalachian piedmont [30]. Immigration to the area began during the mid-1700s.…”

“…Immigration to the area began during the mid-1700s. Most early settlers in the catchment were engaged in general farming practices until after the Civil War (1861-1865) [30]. A typical farm at the time included a combination of free-ranging livestock (mostly pigs), vegetable gardens, and a few fruit trees and/or cornfields.…”

“…In fact, James et al [6] argued that the region, including Big Harris Creek, is now one of the most eroded and gullied agricultural regions in North America. The net result of this upland erosion was the relatively rapid deposition of legacy sediments on floodplains and within the channel bed (Figure 2a,b) [5,27,30,50], two depositional areas that comprise less than 10 % of the total landscape [17,54].…”

“…In general, these legacy deposits can be subdivided into two broad categories, including sediments that accumulated upstream of milldams, which were historically pervasive in the eastern and southeastern U.S. [22][23][24][25][26], and sediments that accumulated on valley floors as a result of extensive upland soil erosion and gully development following widespread land-use change [5,20,[27][28][29]. In the latter case, the formation of legacy sediments was often associated with an aggradational-degradational episode (ADE) that included [30] (1) an extensive period of hillslope and valley bottom clearing, resulting in major changes in land-cover; (2) the formation of upland gullies and accelerated soil erosion which increased sediment loads to the channel; (3) accelerated rates of floodplain deposition and/or channel bed aggradation that produced a thick sequence of highly stratified legacy sediments; (4) a renewed period of land-use/landcover change, including reforestation, that was accompanied by the use of upland soil conservation practices to reduced upland degradation and sediment production; and (5) a phase of channel bed incision that occurred in response to decreased sediment loads and that exposed both precolonial and legacy sediments in the channel banks.…”

“…Moreover, an understanding of the nature, age, and spatial distribution of the deposits upon which the history of a basin's disturbances may be defined is critical to effective management [17]. For example, a river's contemporary sediment and water quality are often influenced by the influx of historical (legacy) sediments (and their associated contaminants) that serve as secondary sources of contamination to the aquatic environment [26,[30][31][32][33][34]. In addition, the stratigraphic deposits produced by human-induced geomorphic events generally possess geotechnical and other properties that differ from older deposits, and these differences can significantly impact contemporary channel processes, form, and restoration [30,[35][36][37].…”