Preliminary assessment of channel stability and bed-material transport along Hunter Creek, southwestern Oregon

Abstract

This preliminary assessment of (1) bed-material transport in the Hunter Creek basin, (2) historical changes in channel condition, and (3) supplementary data needed to inform permitting decisions regarding instream gravel extraction revealed the following: Along the lower 12.4 km (kilometers) of Hunter Creek from its confluence with the Little South Fork Hunter Creek to its mouth, the river has confined and unconfined segments and is predominately alluvial in its lowermost 11 km. This 12.4-km stretch of river can be divided into two geomorphically distinct study reaches based primarily on valley physiography. In the Upper Study Reach (river kilometer [RKM] 12.4-6), the active channel comprises a mixed bed of bedrock, boulders, and smaller grains. The stream is confined in the upper 1.4 km of the reach by a bedrock canyon and in the lower 2.4 km by its valley. In the Lower Study Reach (RKM 6-0), where the area of gravel bars historically was largest, the stream flows over bed material that is predominately alluvial sediments. The channel alternates between confined and unconfined segments. The primary human activities that likely have affected bed-material transport and the extent and area of gravel bars are (1) historical and ongoing aggregate extraction from gravel bars in the study area and (2) timber harvest and associated road construction throughout the basin. These anthropogenic activities likely have varying effects on sediment transport and deposition throughout the study area and over time. Although assessing the relative effects of these anthropogenic activities on sediment dynamics would be challenging, the Hunter Creek basin may serve as a case study for such an assessment because it is mostly free of other alterations to hydrologic and geomorphic processes such as flow regulation, dredging, and other navigation improvements that are common in many Oregon coastal basins. Several datasets are available that may support a more detailed physical assessment of Hunter Creek. The entire study area has been captured in aerial photographs at least once per decade since the 1940s. This temporally rich photograph dataset would support quantitative analyses of changes in channel planform as well as vegetation cover. Light Detection And Ranging (LiDAR) data collected in 2008 would facilitate hydraulic and sediment-transport modeling and characterization of bar elevations throughout most of the study area. Few studies describing channel morphology and sediment transport exist for the Hunter Creek basin. The most detailed study reported channel incision and bank instability as well as the loss of point bars and pools in the lower 3.9 km of Hunter Creek from slightly downstream of its confluence with Yorke Creek to its mouth (EA Engineering, Sci-ence, and Technology, 1998). Repeat channel cross-sections collected from 1994 to 2010 at four bridges indicate that Hunter Creek is dynamic and subject to channel shifting, aggradation, and incision. Despite this dynamism, the channel at three bridge crossings showed little net change in thalweg elevation during this period. However, the channel thalweg aggraded 0.55 m from 2004 to 2008 near the bridge at RKM 3.5. Systematic delineation of gravel bars from aerial photographs collected in 1940, 1965, 2005, and 2009 indicates a 52-percent reduction in the area of bed-material sediment throughout the study area from 1940 to 2009. Net bar loss was greatest in the Lower Study Reach from RKM 1-4 and mainly is associ-ated with the encroachment of vegetation onto upper-bar surfaces lacking apparent vegetation in 1940. Bar-surface material was approximately equal in size to bar-subsurface material at Conn Creek Bar, whereas it was distinctly coarser than the subsurface material at Menasha Bar. Armoring ratios, which indicate the coarseness of the bar surface relative to the bar subsurface, were calculated as 0.97 for Conn Creek Bar and 1.5 for Menasha Bar. These ratios tentatively show that