Assessing flow alteration and channel enlargement due to dam management at Hog Park Creek, Wyoming
Date
2016
Authors
Carleton, Tyler J., author
Fassnacht, Steven R., advisor
Butters, Gregory, committee member
Stednick, John D., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
As part of a complex water exchange agreement, Little Snake River water is piped through the Continental Divide and released into Hog Park Creek to replace over-appropriated North Platte River piped to Cheyenne, Wyoming. The Little Snake River water, in addition to native flows, has used Hog Park Creek as a conduit since the 1960s. As a result, Hog Park Creek has continued to enlarge. This study assesses flow alterations and channel enlargement at Hog Park Creek due to dam management. To assess flow alterations at Hog Park Creek without a pre-dam daily flow record, the Precipitation-Runoff Modeling System (PRMS) simulated natural flows from 1995 to 2015. A regionalization technique transferred calibrated parameters to Hog Park Creek model parameterization from Encampment River model parameterization. Along with the simulated natural flows, reference flows were used to compare to the post-dam flow record. All comparisons indicate the greatest flow alterations were winter and spring monthly flows and low flows. The April median flows and 7-day low flows more than tripled. To a lesser degree of deviation, significant flow alterations included peak flow alterations such as greater magnitude, longer duration, increased frequency, earlier peak flow timing, and faster fall rates. In addition, flow alterations due to climate were assessed. The climate trends reflect warmer-wetter climate change with a shift to earlier peak flows. However, these flow alterations are minor compared to those by dam management. The climate projections compared historic (1980-1999) and future (2040-2059) PRMS simulated natural flows using warmer-wetter and -drier scenarios. Both scenarios project more frequent, flashier peak flows. The warmer-wetter scenario also projects a shift to earlier peak flows. This projected shift of peak flows to mid-May is earlier than the current artificial peak flows in late-May and the natural peak flows in early June. Channel enlargement measured at Hog Park Creek is consistent with qualitative channel response for increased flows and sediment loads less than sediment transport capacity. Stream surveys from 2006 and 2015 measured irregular channel widening and bed degradation. The riffle cross-sections (XSs) measured little change while pool XSs at the maximum point of scour measured extensive widening (+ 3.6 m). Ecologic implications of continued channel enlargement were evaluated by modeling changes in water surface elevations using the Hydrologic Engineering Center River Analysis System (HEC RAS). Between 2006 and 2015, modeling indicated a decrease in water surface elevation by 3 cm per decade and a decrease in flood inundation area of 70 m2 per 1 m of stream length per decade. Additionally, the hydraulic modeling results support the theory that alluvial channel form is most influenced by bankfull flow, which in this case is the 1.5-year flood. Based on this agreement, modeling indicated channel enlargement began near a pre-dam bankfull flow of 3.8 m3 s-1 (135 ft3 s-1) and has since increased to 5.5 m3 s-1 (195 ft3 s-1) in 2015. A possible trajectory of channel enlargement is to a bankfull flow of 5.8 m3 s-1 (205 ft3 s-1), which is based on the 1.5-year flood since dam enlargement in the 1980s. However, without a stable flow regime, a stable channel form is not possible. Thus, to improve aquatic and riparian habitat, a stable flow regime and channel form will be necessary. For this reason, recommendations for a modified flow regime based on the findings of this study are developed and can be used as guidance for adaptive management.
Description
Rights Access
Subject
climate change
flow alteration
trans-basin diversions
dams
channel enlargement
Hog Park