Semester

Spring

Date of Graduation

2020

Document Type

Thesis

Degree Type

MA

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Jamison Conley

Committee Co-Chair

J. Steven Kite

Committee Member

Nicolas Zegre

Abstract

A set of 22 southwestern Pennsylvania streams flowing into the Monongahela and Ohio rivers with drainage areas ranging from 8 km2 to >512 km2 exhibit knickzones, when compared to Mackin’s (1948) idealized graded stream profile. Watersheds delineated from LiDAR-derived 1 m DEMs were processed using Esri’s ArcGIS for Desktop 10.6 to extract stream elevation data used to create stream profiles. These data were exported to MS Excel, where gradients were calculated and compared. This study used a new methodology in which gradient was calculated using incremental measurement distances (d) at every meter along the length of each stream. Profile convexities were identified where downstream reach (d/2d) gradient was greater than upstream reach (d/2u) gradient, and “knickzone extents” were designated where extended stream reaches had downstream gradients ≥.001 and ratios of downstream to upstream reach gradients ≥1.2. A selection of stream profiles was plotted using Goldrick and Bishop’s (2007) DS profile form, revealing most upper reaches fit the graded stream model, but lower reaches show disequilibrium steepening. Disequilibrium was likely induced by the rerouting of the regional drainage to its present configuration by Pleistocene glaciation, based on strong correlation of knickzones elevations with elevations of Pre-Illinoian and Illinoian terrace features along the main trunk rivers. The best correlation occurred when study area streams were compared by confluence location along the main trunk, possibly due to the lithologic differences in bedrock across the area or due to differences in the evolution of the Allegheny and Monongahela rivers. Although knickzones were assumed to have migrated into study area tributaries from the main trunk, migration distance appears to be controlled more by lithology or pre-incision topography rather than time elapsed since inducement of disequilibrium.

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