COMPLEX GEOMORPHIC LEGACIES OF DISTURBANCE IN A DYNAMIC EOLIAN-FLUVIAL CHANNEL NETWORK

Abstract

North Sand Creek is a tributary of the North Platte River draining the southern Medicine Bow mountains in the northeastern corner of North Park, Colorado. Its perennial, montane segment upper meets the creek’s ephemeral tributary, which flows adjacent to the active dune field of North Sand Hills. Below this confluence, the stream runs alongside inactive, vegetated dunes, then forms a low-slope, sandy alluvial fan, where State Line Ranch diverts North Sand Creek for irrigation. Previous work identified high sediment loading caused by recreational off-highway vehicles (OHVs) on the active dunes, which generates waves of sediment and channel instability that impair State Line Ranch’s protected downstream water right. My observations of relict beaver modifications in the stream corridor suggest that channel instability and sediment export may be related to historical beaver extirpation. My objectives in this study were to: (i) compare channel dynamics between the 2019 and 2025 runoff seasons, (ii) map potential sediment sources downstream from the main, highly active dune face on the unnamed tributary and examine potential correlations between stream corridor morphology and the location of these sediment sources, (iii) estimate the upper and lower bounds of potential sediment storage associated with beaver dams based on the location and characteristics of relict beaver dams, (iv) compare the surface area and recent history of sediment deposition and channel change on the alluvial fans of North and East Sand Creeks and adjacent catchments along the eastern side of North Park, and (v) provide guidance for restoration actions in North Sand Creek based on my results. Repeat cross-section surveys, mapping of relict beaver dams and sand addition points, and collection of high-resolution UAV-derived imagery and surface elevation date were completed during summer 2025. I found that (i) a sediment wave did not occur during 2025, demonstrating interannual variability and annual-scale complex response in sediment transport and storage; (ii) discontinuous sand addition to the perennial mainstem from destabilized dune faces and reworked terrace deposits occurred along only 7.5% of total bank length; (iii) over 300 relict beaver dams potentially created 10,200 – 67,300 m3 of sediment storage at estimated historical density, and local beaver extirpation in the 1970s caused entrenchment, simplification, and loss of attenuation capacity in the stream corridor; (iv) the North Sand Creek alluvial fan is an outlier in estimated volume per drainage area, indicating high historical sediment export; and (v) North Sand Creek has incised 4 m since 2830 yBP, leaving a terrace elevated above the modern valley floor. North Sand Creek’s current planform, valley cross-section, and instability are the result of Late Holocene incision, followed by further historic entrenchment and loss of attenuation capacity after beaver extirpation, and exacerbated by OHV introduction of sand to the channel. Restoration practitioners and managers working in North Sand Creek must (i) acknowledge that mass sand export is a long-term process that cannot be stopped, but rather only attenuated; and (ii) target root causes of channel instability by reducing OHV use along the ephemeral tributary and rebuilding lost attenuation capacity through careful use of channel and floodplain structures, and perhaps eventually returning beaver to the landscape. This study provides a novel description of historical beaver modifications to a highly variable eolian-fluvial system and contributes to the body of literature describing complex response across diverse environments and time scales.