Waiting on a mule deer crossing…
Unaweep Canyon is a large valley that bisects the Uncompahgre Plateau of western Colorado but hosts no through-flowing axial stream; rather, two underfit creeks flow in opposite directions from a subtle divide within the canyon’s valley-floor. Widely recognized as enigmatic, formation of Unaweep Canyon remains debated, with hypotheses ranging from late Cenozoic fluvial incision to Pleistocene glaciation, to late Paleozoic glaciation and late Cenozoic fluvial exhumation followed by partial burial and abandonment. Thick Quaternary sedimentary fill in the valley floor, and Mesozoic strata capping the plateau obscure much of the Precambrian-hosted bedrock geometries of Unaweep Canyon, complicating morphometric assessment and precluding analyses such as cross-sectional interpretation and power-law model fittings. To address this challenge, we conducted quantitative basin morphometric analyses of 47 Precambrian-hosted tributaries within a ~37 km long section of Unaweep Canyon, comparing results against analogous analyses of Black Canyon of the Gunnison (fluvial control, n=122) and Yosemite Valley (glacial control, n=18). Statistics moments derived from hypsometric curves, including the hypsometric integral (HI), skewness (SK), kurtosis (KU), density skewness (DSK), and density kurtosis (DKU), along with Inflection Point (I) metrics (Eh and h*) were computed from a novel truncated, rescaled ‘mid’ hypsometric curve (0.1-0.9 h/H) designed to isolate Precambrian-dominated tributary morphometries from the noise introduced by Quaternary valley-floor sediments and Mesozoic rim strata. Additional metrics including tributary summary statistics (area to relief ratio and log-transformed volume and relief), the Hypothetical Baseline Equilibrium Elevation (HBEE) – a proxy of a glacier’s equilibrium line altitude that links present-day topography to possible past glaciations--, and the basin shape (BS) index were incorporated into a 12-variable principal component analysis (PCA) and average linkage cluster analysis. Unaweep Canyon was divided into two separate sections for analysis, with West Unaweep tributary basins representing wider morphometries than the narrower basins of East Unaweep Canyon. Additionally, unlike the amphitheater-shaped tributaries of western Unaweep Canyon, the dendritic and eastward-draining morphometries characterizing the tributaries of eastern Unaweep Canyon mimic the drainage pattern of the greater eastern Uncompahgre Plateau.PC1 (~43% variance), driven by hypsometric maturity metrics (HI, SK, KU, DSK and Eh) and PC2 (~16% variance), driven by relief, HBEE, area to relief (AR) ratio, volume, and BS index, both group West Unaweep tributaries with Yosemite’s tributaries, and East Unaweep tributaries with Black Canyon’s, indicating that tributary morphometries of West Unaweep Canyon reflect greater erosional maturity along with volumetric characteristics consistent with glacial erosion. The cluster analysis strongly supports a two-group separation between West Unaweep and Black Canyon, with PCA-backed interpretation suggesting a stronger clustering of Yosemite with West Unaweep rather than with Black Canyon. Results suggest that the Precambrian-hosted tributaries of West Unaweep Canyon are most consistent with a glacial origin. These findings represent the first application of quantitative tributary basin morphometric analysis to a potential deep-time paleovalley, and the novel ‘mid’ hypsometric approach introduced here offers a transferable method for morphometric study of ancient, sediment-laden landforms.
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