Response of the Pedernales River of Central Texas to Late Holocene Climatic Change

Bibliographic Data

ID8374759
AuthorsMichael D Blum (0000-0003-2306-0914, The University of Texas at Austin), Salvatore Valastro (0000-0002-1297-4174, Balcones Technologies (United States))
Year1989
Volume79
Issue3
Pages435-456
Publication date1989-09-01
Peer ReviewedYes
Open AccessNo
TypeARTICLE
VenueAnnals of the Association of American Geographers (JOURNAL)
Journal identifiersISSN: 0004-5608 • E-ISSN: 1467-8306
PublisherInforma UK Limited (PUBLISHER • GB)
DOI10.1111/j.1467-8306.1989.tb00271.x
OpenAlexW2028628075
LanguageEN
Citations received21
References cited40

Geomorphological investigations of modern fluvial systems are based on the premise that stream channels develop an equilibrium morphology that represents an accommodation to the relationship between the independent variables of discharge and sediment supply. Long-term changes in this relationship, often due to climatic change, result in morphological and sedimentary adjustments by the fluvial system that are commonly recorded in alluvial sedimentary sequences. The Pedernales River is a bedload-dominated stream that drains the eastern margins of the Edwards Plateau in Central Texas. In the upper third of the valley, there is a complex alluvial sequence that offers considerable insight into the long-term evolution of this largely erosional landscape. This paper is concerned with the detailed and temporally well-defined late Holocene record, and provides an example of widespread morphological and sedimentary adjustments to subtle changes in climate. Through much of the late Holocene, from ca. 4500 to 1000 yrs B. P., the Pedernales River was an aggrading, gravel-dominated meandering stream, with a bankfull width-to-depth ratio of about 35. Channel incision and flood plain abandonment, accompanied by systematic adjustments in channel geometry and sedimentation style, occurred ca. 1000 yrs B. P.: the modern Pedernales channel is underfit with respect to the late Holocene meanderbelt, and maintains a bankfull width-to-depth ratio of 12, with a finer-grained, gravel-poor depositional system. Paleoenvironmental data suggest the climate prior to ca. 1000 years ago was more moist than today, whereas the sedimentological record from the Pedernales River indicates that the rainfallrunoff regime delivered an overabundance of coarse gravel from the tributary network to the valley axis. By contrast, the rainfall-runoff regime of the last millennium, presumed to be similar to average conditions of today, has supplied little coarse gravel to the trunk stream. Several conclusions illustrate the sensitivity of fluvial systems to climatic change and the variability of response. In contrast to the wide-spread occurrence of underfit channels that developed following the end of the Pleistocene, the modern Pedernles channel is underfit with respect to meander wavelengths that were established during the late Holocene period of channel aggradation and flood plain construction. In addition, previous discussions of morphological and sedimentary adjustments to climatic change have emphasized that more arid time periods are characterized by a coarser-grained sediment load, with channels that maintain a low sinuosity and high width-to-depth ratio, whereas more humid periods result in finer-grained sediment loads, and channels that are more sinuous but relatively narrow and deep. In the Pedernales drainage, the valley axis was dominated by a coarse gravelly sediment load under a more humid climate prior to ca. 1000 yrs B. P., and maintained a moderately sinuous channel with a broad, shallow cross-section. Transition to the drier climate of the last millennium was associated with a textural fining of the sediment load, a small increase in sinuosity, and a large decrease in channel width-to-depth ratio. The results also caution against interpretation of alluvial sedimentary sequences based on explanatory models that were developed in regions with different systemic controls and over different time spans

Aggradation · Alluvial fan · Alluvium · Archaeology · Channel (broadcasting) · Climate change · Flood myth · Fluvial · Geography · Geomorphology · Holocene · Physical geography · Sediment · Sediment transport · Sedimentary budget · Sedimentary depositional environment · Sedimentary rock · Sinuosity · Stream power · Structural basin · Geological formations and processes · Geology · Hydrology and Sediment Transport Processes · Oceanography · Paleontology · Soil erosion and sediment transport

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