Knickpoint migration and landscape evolution, Cullasaja River Basin, North Carolina

Introduction:
Localized high topographic relief, steep slopes, and frequent mass wasting — features commonly associated with tectonically active settings — characterize the landscapes of the southern Appalachians. However, the Appalachians are an orogen that has been tectonically quiescent for more than 200 million years. The fundamental processes responsible for these characteristics remain unclear. With a relatively uniform lithology, no evidence of late Pleistocene alpine glaciation, and a position more than 500 km south of the last glacial maximum limit, the Cullasaja River basin in western North Carolina, serves as a natural laboratory absent of many factors that may complicate studies aimed at elucidating causes for such a rugged and active landscape (Figure 1). We show that an investigation of landscape morphology using digital topography coupled with a geographic information system (GIS) in the Cullasaja River basin points to knickpoint migration as a way to explain the present-day activity of the southern Appalachians, and may help answer fundamental questions about landscape evolution in post-orogenic settings, in general.

Hypothesis:
The Cullasaja River basin and its tributaries are observed to contain numerous knickpoints. A knickpoint is simply defined as an over-steepened section of a river or stream relative to adjacent reaches (a waterfall is a good example) and are a mechanism by which rivers incise. Many knickpoints migrate upstream communicating a signal of local base level [base level is the lowest elevation to which a river can flow often times defined by its outlet of mouth and for large rivers sea-level] fall to the surrounding landscape (e.g. Howard, 1994; Bigi et al., 2006; Crosby and Whipple, 2006). Because it is fluvial incision that initiates the landscape response to a local base level fall, in the wake of the passage of a knickpoint there is often a delayed and fleeting response along hillslopes adjacent to channels as they equilibrate to the newly imposed local base level (Figure 2). Topographically this signal may be manifested as locally high hillslope gradients [slope] and local relief [the difference between the maximum and minimum elevations over a given area] along hillslopes just downstream of a migrating knickpoint. These changes in topographic form will likely result in increased rates of hillslope denudation [the sum of all processes acting to lower the elevation of a landscape (i.e. weathering and erosion) over a given length of time]. The Cullasaja River contains three large knickpoints, known as Cullasaja, Dry, and Kalakaleskies Falls that are presumably linked to knickpoints found along downstream tributary channels, and are thus responsible for communicating the signal of baselevel fall to the surrounding landscape (Figure 3). We hypothesize that it is the upstream migration of knickpoints are related to the locally high relief, steep slopes, and mass wasting events [that are more likely to occur on steeper slopes] typical of the Cullasaja River basin specifically and the southern Appalachians in general (Gallen et al., 2011).

Experiment:
High-resolution (6m-horizontal) bare earth airborne lidar and a digital inventory of recent, historic, and mapped prehistoric landslides are available for the Cullasaja River basin. These datasets combined with a GIS allow for the testing of the hypothesis that there is a predictable spatial relationship between knickpoint position on the trunk channel and variations in hillslope topographic form (e.g. gradient and relief) and the frequency of landslides (a form of denudation). To test this idea we extracted data from individual tributary basins from the headwaters to the mouth of the Cullasaja River.

If true, our hypothesis predicts that downstream from major knickpoints along the Cullasaja River, tributary basin hillslopes will have greater relief, gradient (slope), and frequency of landslides than those upstream of the same knickpoints. Larger hypsometric integrals (HI) [hypsometry is the proportion of land area per unit elevation] are also expected for the tributary basins downstream of major knickpoints as the HI is commonly regarded as a measure of the erosional activity of a landscape because it is thought to positively correlate with erosion rates (Strahler, 1952). HI is estimated by:

HI = (Z_o – Z_min)/(Z_max – Z_min)

where Z_o, Z_min, and Z_max are the mean, maximum, and minimum elevations, respectively, for an entire tributary basin with higher numbers correlating to a more erosionally active landscape (Pike and Wilson, 1971). If our hypothesis is incorrect then random variability in all topographic metrics and landslide frequency is expected.

Results:
Metrics of relief, mean slope, HI, and landslide frequency per km² [normalized to area of the basin] from 14 tributary basins along the Cullasaja River show considerable, but not random, variability from the river headwaters to its mouth. In tributary basins above major knickpoints relief, mean slope, and HI remain relatively low (Figure 4). Just downstream of these knickpoints, however, significant increases in all topographic metrics are observed and begin to decrease near the river mouth (Figure 4). Normalized landslide frequency results exhibit a more scattered but still not random headwater to river mouth pattern. In general normalized landslide frequency is low in tributary basins above major knickpoints and increases approaching the Cullasaja river mouth (Figure 4). This evidence suggests that hillslopes are responding to fluvial forcing not only in changes of topographic form but also process (e.g. increased rates of denudation).

Interpretation:
These results suggest that the contemporary high relief and steep slopes of the southern Appalachians are the result of a series of local base-level falls induced by the passage of knickpoints, of unknown origin. Evidence for the Cullasaja River basin also implies that the landscape responds to these changes in topographic form presumably by increased rates of hillslope denudation often manifested as an increased frequency of mass wasting events. Further research is needed to quantify the drivers behind knickpoint formation, their retreat rates, and possible spatial and temporal changes in the rates of hillslope denudation throughout the Cullasaja River basin specifically and post-orogenic mountain ranges in general.

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