Graduate Thesis Or Dissertation
 

Long-Term Stream Channel Response to a Large Flood in a Forested Mountain Watershed

Public Deposited

Downloadable Content

Download PDF
https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/hq37vw262

Descriptions

Attribute NameValues
Creator
Abstract
  • There is still scholarly debate on the impacts of large floods on the geomorphic evolution of mountain rivers. Understating the geomorphic effects of large flows in mountain rivers is challenging given the hydraulic complexity of these systems and the inherent unpredictability of large floods. Prior work has demonstrated that extreme floods in mountain regions can result in changes to channel form, the magnitude and direction of which varies not only as a function of the size of the extreme event but also on depending on local channel characteristics. Despite this variability, few long-term studies have explored the impacts of large floods. Cross sections were repeatedly surveyed from 1978 to 2011 within four, 3rd–5th order stream reaches in the HJ Andrews Experimental Forest in the Cascade Range, Oregon, USA. In February of 1996, the study site experienced mass movements and flooding in response to a storm which broadly impacted the Pacific Northwest region. At the lower gaging station, the peak flow of 1996 is the flood of record which was 290% greater than the mean annual peak flow. Repeat cross-sectional surveys showed that the flood of 1996 produced disproportionately greater effects than any moderate flood that occurred during the monitoring period. After 17 years of limited very minimal channel change in the pre-flood period, the 1996 flood induced measurable changes in sediment storage, bankfull channel geometry, and channel grain size at all of the study reaches. However, the magnitude and direction of these changes varied from reach to reach. Changes to channel form also varied within reaches. A low-gradient (1.5%), partly confined reach experienced scouring and deepening near its channel bend and deposition and depth reduction downstream of the bend. A medium-gradient (3.2%), unconfined reach experienced deposition, fining, and bankfull depth reduction upstream of a large logjam. Many of these changes persisted through the end of monitoring in 2011. Our findings suggest large, debris-laden flows may play a fundamentally different role in fluvial adjustment than smaller, lower density flows. The placement and stability of channel-spanning logjams also affects flood response. In contrast to the other long-term monitoring sites in the western US, stream channels at this site appear relatively quiescent due in part to their more resistant lithology. While the 1996 flood produced a decadal-scale impact on channel geometry in many parts of the study reaches, grain size adjustment occurred on the 1 to 5-year time scale, which suggests that mountain streams might return to their pre-flood sediment transport conditions much more quickly than they return to their pre-flood cross-sectional geometries.
License
Resource Type
Date Issued
Degree Level
Degree Name
Degree Field
Degree Grantor
Commencement Year
Advisor
Committee Member
Rights Statement
Publisher
Peer Reviewed
Language
Embargo reason
  • Pending Publication
Embargo date range
  • 2020-09-21 to 2021-10-21

Relationships

Parents:

This work has no parents.

In Collection:

Items

Thumbnail Title Date Uploaded Visibility Actions
file details GoodmanAriannaC2020.pdf 2020-09-20 Public Download