Date of Thesis

Spring 2026

Description

Scroll bars, arcuate sediment ridges found along the inner banks of river bends, are common floodplain features in meandering river systems that form during lateral channel migration. Two hypothesized formation mechanisms described in prior studies are the aggradation of suspended load onto abandoned inner bank levee (IBL) deposits during overbank flow and the migration of transverse bar packages from the channel bed up onto the point bar during flooding events. To study the sedimentology and geometry of these features, we conduct remote sensing and field analysis on a 60 km portion of the Kantishna River, AK: a highly sinuous, pro-glacial meandering river with laterally migrating chute channels. Here we present results from two field seasons, with key goals including: 1) sedimentologically characterize scroll ridges to better understand depositional processes; 2) quantify the geometry and spacing of scroll bars, to test the hypothesis that process origin controls ridge geometry; and 3) compare scrolls in main and chute channel reaches to understand the role of channel size on scroll properties.

Results from fieldwork indicate that there are two prominent scroll bar types with distinct genetic processes in the Kantishna River system: unit bars and IBL deposits. IBL deposits are dominated by repeated packages of channelward-dipping, upwardly fining silty fine sands. Unit bars are composed of coarse to silty medium cross-bedded sand, with troughs oriented obliquely downstream. Over 80% of scroll ridges in the vegetated floodplain are IBL deposits.

We use field measurements and Google Earth satellite imagery of the Kantishna River system to map scroll ridge geometry, including length/width ratio (L/W). Results 2 show that each scroll bar type has a characteristic geometry: IBL-type scrolls form long, narrow ridges in cohesive packages (L/W: 23.86), while unit bar-type scrolls form wider, shorter ridges (L/W: 5.28). Using this framework, potential unit bar deposits have been identified in additional reaches of the Kantishna, and in the Mississippi River floodplain. Additionally, our analysis shows that the main and chute channel reaches of the Kantishna River create indistinguishable scroll bars, with no significant difference in grain size distribution, vertical relief, spacing, or ridge type. This suggests that local channel width and discharge are not key controls on scroll bar formation; rather, system-wide variables dominate, such as flood periodicity and vegetation growth rates controlling ridge stabilization.

Keywords

fluvial geomorphology, scroll bar, unit bar, levee

Access Type

Honors Thesis

Degree Type

Bachelor of Science

Major

Geology

Second Major

Classics & Ancient Mediterranean Studies

First Advisor

Ellen Chamberlin

Second Advisor

Jeffrey Trop

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