Degradational Stream Deposits of the Brazos River, Central Texas

Abstract

According to a geomorphic investigation, the Brazos River in a 250-mile segment of its interior valley (Waco to Knox City, Texas) is a degradational stream with diverse channel patterns and unevenly distributed alluvium, and the character of the alluvium indicates that valley deepening was also a dominant trait of the stream during the Pleistocene. This is indicated chiefly by several diagnostic properties of the alluvium which seem to be directly related to stream degradation acting concurrently with lateral planation and accretion. These properties serve as an excellent basis for interpreting the history of channel migration. The erodibility of various pre-Tertiary strata, into which the Brazos is presently incised, accounts for differences in stream gradient, channel form, and sedimentary load and largely determines the type of stream pattern developed. Typical patterns of braiding and flood-plain meandering are confined respectively to soft bedrock in the distal upper and lower parts of the valley; incised meanders are developed in resistant strata of the lengthy intervening segment. Alluvium which expands to form prominent terraces in the more erodible valley segments (two terraces in the upstream segment and three in the downstream segment) indicates extensive lateral planation of the stream during downcutting of soft bedrock. Four diagnostic properties of the alluvium point to such a history: (1) the deposits decrease upward in component size from gravel to terminal silt or clay and represent the successive accumulation during channel migration of bed load, bank, and overbank-flood-plain deposits; (2) the average thickness of the alluvium, about 30 feet, is a function of stream-stage variation and equal to the vertical distance between channel bed and flood plain; (3) the terraces and underlying bedrock surfaces have two components of slope: a major cross-valley component related to lateral shifting of the stream and concurrent downcutting and a minor down-valley component imposed by the stream gradient, and (4) the terraces are normally unpaired because lateral planation and bank undercutting have precluded the formation of opposing valley-side counterparts. Interpretations based on these diagnostic properties could have wide application, because the properties should be typical of degradational stream deposits in general. Several lines of evidence besides that of the alluvium indicate that the “Seymour beds” and younger alluvium immediately west of Seymour, Texas, are lateral accretions of two streams — the northerly shifting Brazos and a westerly shifting former tributary of the Brazos. From the slope of stream-planed bedrock surfaces beneath the alluvium, successive channel positions of the two streams are reconstructed and show the pattern of shifting of the tributary, which eventually led to its diversion into the Wichita River system. The limited distribution of lenses of Pearlette ash in the Seymour indicates the approximate position of both streams at the time of a far-reaching ash fall during the middle Pleistocene (late Kansan or early Yarmouthian). According to these indicated positions, the bulk of the alluvium was deposited before the ash fall.