The Signal of Modern to Holocene Drivers of Complex Channel Response of a Small Alluvial Stream
Authors: Ducar Scott D. 
Published: February 2021

Small alluvial streams (~100km2 drainage area) are important for water resources and aquatic habitat. Small streams throughout the Western United States are impacted by anthropogenic land-use including urban development, mining, logging, beaver trapping, grazing, and farming. Land-use change can trigger a complex series of channel response (such as stream channel incision or channel migration) that vary spatially and temporally in the watershed. However, streams also respond to other external forcings, such as tectonically or climatically-driven changes in discharge or base-level, which make disentangling the drivers of channel response complicated. Therefore, it is important to place modern channel changes into a longer geomorphic context to fully understand the complex response initiated by land-use. In order to understand how changes in land-use may drive spatially variable channel response, we examine a representative small alluvial stream, Lower Dry Creek (LDC), a tributary to the Boise River in Idaho.

LDC marks the transition from the rugged and largely undeveloped upland Dry Creek Experimental Watershed to the lower gradient, agricultural, and residential section of the watershed. LDC has a complex history of placer mining, beaver trapping, grazing, and farming since the 1850's. Recent (post-1997) growth in the region converted LDC's expansive floodplain from agricultural land to housing developments. Most of the recent development and historic and current farmland are on the broad, low gradient Hidden Springs Terrace. We use remote sensing, hydraulic modeling, grain size analysis, and field observations to quantify how the distinct reaches of LDC are changing over human time scales; we use Quaternary dating methods and geomorphic mapping to examine how LDC has changed over centennial to millennial time scales.

Optically Stimulated Luminescence (OSL) dates of fluvial sediments in an upper reach indicate incision in LDC after 4.79 ± 1.05 ka. Around 0.79 to 0.67 ka, LDC deposited a large packet of sheetfloods and cross-bedded sands, which correlate to a period of more fire activity and alluvial fan deposition in the region. After approximately 0.67 ka the reach incised 2.4 m. In the late 1800's, placer mining in the upper reach of LDC shifted the channel behavior from incision to lateral adjustment. We measured an average of 0.6 m/yr of meander migration from 1938 to 2019. Migration rate increased threefold after 1992 (which corresponds temporally with a large rain-on-snow flood event in 1997), but slowed after 2011.

Comparison of the modern longitudinal profile of LDC with the longitudinal profile of the Hidden Springs Terrace, combined with grain size analysis and historic dating reveals the impact of prior land use change on the present channel. LDC's current profile is convex in middle reaches, and grain size analysis shows a fining in the middle reaches and then coarsening downstream. The convexity and grain size change is consistent with increased aggradation from a slug of sediment from upstream placer mining progressing downstream. Downstream where the valley is unconfined, LDC aggraded 0.75 cm/yr from 1642 to 1950 AD to form the broad Hidden Springs Terrace.

Notably, a lower reach of LDC has recently and profoundly incised, affecting local landowners and cutting off access of the stream to its floodplain. This downstream reach is incised 4.7 m below the Hidden Springs Terrace; a modern radiocarbon date provides evidence the incision happened post-1950 AD, potentially from channelization of Currant Creek (a tributary of LDC) as farmland is converted to housing. Hydraulic modeling shows LDC's median grain size is mobile at estimated bankfull flows for all reaches, which allows the stream to rapidly adjust both vertically and laterally.

LDC channel response in the upper and lower reaches indicates anthropogenic land-use resulted in vertical and lateral channel change: upstream aggradation and meander migration following placer mining, and downstream incision following farmland conversion. This represents a shift from the observed channel adjustments and large-scale formation of the Hidden Springs Terrace observed over Holocene timescales. LDC illustrates textbook "complex response" as the stream both incises and aggrades in different locations due to differing drivers.

This study shows small alluvial streams can be very sensitive to changes in land-use. Stream incision, aggradation, and channel shifts impact aquatic and riparian species and developments adjacent to the channel. This study illustrates the importance of examining the drivers of modern channel change within a longer more complete context. Results of this study can support stakeholders as they strive to understand the characteristics and response of small alluvial streams to anthropogenic land-use, and best options for restoration of degraded systems.