Late Cenozoic (post-3.6 Ma) entrenchment of the Rio Grande in northern New Mexico formed a gorge over 200 m deep and induced different types of fluvial adjustments on two adjacent tributaries. Rio Hondo (drainage area 181 km2 ) has a concave-up long-profile and flows in a narrow valley containing six Quaternary strath terraces with parallel long-profiles grading to the Rio Grande. In contrast, Rio Pueblo de Taos (drainage area 1080 km2 ) has a convex-up long-profile and flows in a wide valley containing four strath terraces which do not grade to the Rio Grande. Although smaller in drainage area, Rio Hondo has adjusted to base-level fall by continued entrenchment, whereas Rio Pueblo de Taos has produced a 140 m-high nickpoint.
Differences between the two watershed's present-day hydrologic parameters relate to source-area lithology and apparently explain differences in long-term fluvial adjustments. The Rio Hondo watershed is developed in gneiss and granite, and produces higher unit discharge and higher bankfull discharge than the Rio Pueblo de Taos watershed, which is underlain by sandstone and shale. Stream power and shear stress data suggest that higher bedload transport rates and larger bedload grain sizes exist on Rio Hondo during bankfull discharge. These differences coupled with the distinct valley morphologies suggest that, over long periods of time, Rio Hondo has had sufficient stream power to transport its bedload, erode vertically, and keep pace with downcutting on the Rio Grande. Rio Pueblo de Taos has apparently expended its energy via lateral migration rather than vertical erosion. Alternating periods of incision and lateral erosion on both streams may also be linked to hydrologic changes resulting from climatic changes. The present-day hydrologic parameters of these two watersheds suggest that bedrock lithology strongly influences: 1) the duration and magnitude of discharge events, 2) stream power, 3) sediment load, and 4) the relative sensitivity of each basin to base-level change.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Stephen G. Wells
Second Committee Member
Leslie D. McFadden
Third Committee Member
The Student Research Allocation Committee of the University of New Mexico provided partial funding
Kelson, Keith Irvin. "Long-Term Tributary Adjustments To Base-Level Lowering, Northern Rio Grande Rift, New Mexico." (1986). https://digitalrepository.unm.edu/eps_etds/296