Four species of net-spinning caddisflies, Hydropsyche simulans Ross, Cheumatopsyche lasia Ross, Cheumatopsyche campyla Ross and Chimarra obscura (Walker) are common in the regulated portions of the Brazos River. Hydropsyche simulans spun capture nets with the largest meshdimensions; the two Cheumatopsyche species' nets had the next largest meshes, and Chimarra obscura spun nets with the smallest dimensions. Cheumatopsyche lasia and C. campyla constructed nets with similar sized meshes. The number of individuals m~2 and standing crop biomass were not significantly different among low, medium, and high velocities. Early hydropsychid instars fed on detritus while later instar H. simulans and C. campyla had larger proportions of animal material. Guts of later instar C. lasia individuals had a greater percentage of algae and diatoms.

Bottom sediment from a 750-square-mile area offshore from the mouth of the Brazos River, Texas, has been analyzed statistically to determine the pattern and processes of sedimentation of the neritic environment and of a modern delta. The Brazos delta is characterized by topset beds of poorly sorted laminated sand, silt, and clay, foreset beds dipping at approximately 1° composed of fine sand grading downward (seaward) into clay, and poorly developed bottomset beds representing slow deposition of clay from the Brazos combined with reworking of material from a submerged Pleistocene deltaic plain. Interpretation of the sediment on the basis of its modal characteristics indicates that effective sorting by waves develops a unique uniform distribution of sediment types from the beach to a depth of 60 feet. Farther offshore from this depth exotic agents such as hurricanes and uncharted bottom currents are predominant and act to bring "obstacles" (topographic irregularities) into a marine profile of equilibrium. Maps of sediment types, mean size, inclusive standard deviation, inclusive skewness, and kurtosis precisely describe the geologic history of the surface sediments. Individual and bivariant plots of the statistical parameters are shown to be useful in determining direction to the shoreline as well as completely defining the modality of the sediment, which in effect defines the environment of deposition. Fluctuations in source area are reflected by the relation of mean size and depth. Heavy mineral distribution shows that the suite of durable minerals carried by the present Brazos River is diluting an existing widespread suite of less durable minerals characteristic of the Colorado River drainage area. Variation in clay mineral composition results from differential sedimentation and reflects source area, providing no evidence of alteration of the clay minerals during deposition by diagenesis. / text

Thesis (M.S.)--Texas A & M University, 2003. / "December 2003." Includes bibliographical references (p. 56-64).

This study aims to document the dinoflagellate cyst biostratigraphy and paleoenvironmental record of the Maastrichtian Neylandville and Corsicana Formations and the lower part of the Danian Kincaid Formation from the Brazos River, Texas. Rock samples are exposed to standard palynological methods for biostratigraphic interpretations. The quantitative data collected from palynological samples are combined with δ13C and δ18O stable isotope geochemistry and TEX86 and BIT Index organic geochemistry data for paleoenvironmental interpretations. Biostratigraphically important species of dinoflagellates divide the section into three intervals. Interval 1 occurs within the Neylandville Formation, and the presence of Alterbidinium acutulum, Xenascus ceratioides and Isabelidinium cooksoniae indicate that this interval is not younger than early Maastrichtian. Interval 2 represents the Corsicana Formation. The presence of the late Maastrichtian species Disphaerogena carposphaeropsis, Palynodinium grallator and Deflandrea galeata at the base of the Corsicana Formation indicate that this interval is of late Maastrichtian age. Interval 3 occurs within the Kincaid Formation. The presence of Carpetalla cornuta and Damassadinium californicum at the base of the Kincaid Formation indicates that this interval is of Danian age. Previously published Gulf of Mexico palynology studies, as well as planktonic foraminifera and nannoplankton data confirm the age assignments of the studied interval. Dinoflagellate species assemblages increase in diversity upwards from Interval 1 to Interval 2, and then show a small decrease above the K-Pg boundary within Interval 3, indicating that the K-Pg event was not catastrophic for the dinoflagellates. The Cerodinium spp. and Spiniferites spp. complex comprise a large proportion of the species within the section. High abundance peaks of Glaphyrocysta spp., Cribroperidinium spp., and Yolkinigymnium lanceolatum occur within Interval 2. Two intervals in the section are dominated by peridinioid dinoflagellates, measured by the Peridinioid/Gonyaulacoid (P/G) ratio. The first one occurs within Interval 2 and contains peaks of the P/G ratio that correlate with increases in δ13C, suggestive of an increase in paleoproductivity. Two more peaks occur within Interval 3. Bottom water δ18O temperatures determined from benthic foraminifera and sea surface temperatures determined from TEX86 organic geochemistry show an overall cooling trend from Early Maastrichtian to the K-Pg boundary.

Larval, pupal and adult samples of Culoptila cantha, from a large riffle of the Brazos River in north-central Texas from January, 1995 to March, 1997, indicated a predominately trivoltine cycle during both years; the over-wintering generation spanned 6-7 months and warm-season generations spanned 2-3 months. Eggs, larvae of all instars, larval cases, case reconstruction progression and behavior, pupae, and adults are described.

The objective of this study was to determine the kinds and numbers of food organisms consumed by Corydalus cornutus, and to investigate its periodicity of feeding during the spring months in the Brazos River, Palo Pinto County, Texas. A total of 468 larvae were collected at two-week intervals from February 29 to May 14, 1971. Larvae were taken at opportunity during three periods of the day, 6am.-12pm., 12pm.-6pm., and 6pm.-12am. A total of 23 different food items was observed in the 121 stomachs containing food. An expedient method of dissection of the digestive tract is included.

Stability of the macrobenthic community in a regulated section of the Brazos River, Texas, was evaluated. Physicochemical data and information on spatial distribution of macrobenthos were collected. Populations of two invertebrate species, Isonychia sicca and Neoperla clymene, had been greatly reduced since 1970-71 and I. sicca had been practically eliminated from this section of the Brazos. Three other insect populations, Choroterpes mexicanus, Cheumatopsche campyla and Cheumatopsyche lasia, had more than doubled their numbers since the 1970-71 study. A physicochemical gradient existed in this regulated section of the Brazos but it appeared to have changed little in comparison of earlier chemical data. A gradation of the macrobenthic community was evident as distance from the dam increased,

Quantification of stream macrobenthos populations has remained a perplexing problem in rivbrine ecology, despite numerous attempts at improvement. This is in part due to well documented variations in chemical and physical parameters locally and geographically, and resultant adapted macrobenthos populations. Southwood (1968) and Hynes (1970a) have reviewed the various sampling techniques developed'for the census of lotic macrobenthos populations. Needham and Usinger (1956), Chutter (1969), and others have pointed out the difficulty in obtaining adequate numbers of samples which will yield population estimates with desired statistical confidence, and still maintain some degree of sampling economy. Needham and Usinger (1956) and Gaufin et al. (1956) mentioned the "patchy" distribution of aquatic insect populations as the primary source of this difficulty. The concept of patchy distribution in insect populations was originally discussed by Andrewartha (1961). Attempts to improve confidence through improved sampling devices and techniques have led to development of numerous types of samplers. Cummins (1962) indicated that there were almost as many samplers as there were researchers.

The life history and ecology of Neochoroterpes mexicanus was studied from data collected September, 1971, to August, 1972, and January to December, 1973, in the Brazos River, Texas. Nymphal development, instar analysis, voltinism, standing crops, and production estimates were determined from the quantitative samples taken in 1971 and 1972. Aspects of the life history and food habits of 230 specimens were arrived at from qualitative samples and light box captures in 1973. Laboratory investigation in 1973 helped in establishing instar analysis, egg incubation and description, and first instar descriptions. Neochoroterpes mexicanus appeared to have three generations per year with brood overlap in the summer and fall. It displayed 16 and 19 instars for overwintering and combined summer generations respectively.

The Cretaceous–Paleogene (K/Pg) boundary marks one of the major crises in the history of life on Earth. The cause is widely regarded as a large bolide impact at Chicxulub, Mexico, coincident with a major series of volcanic eruptions on the Deccan Plateau, India. Fieldwork in the Brazos River area of Texas has involved an investigation of the sections on the Brazos River and its tributaries. A previously overlooked K/Pg section (RBS) on the Brazos River was found and contains the most accessible and complete K/Pg boundary succession in the area. The RBS succession provides a clear exposure of the various lithological units within the Paleocene and was used to correlate to the successions in the nearby creeks. The K/Pg boundary is also well-exposed and records an erosional relief of ~1 m, cut into the Maastrichtian mudstone succession, creating a mounded topography. The overlying ‘Event Bed’, containing reworked impact spherules at its base, is shown to infill troughs on this irregular surface. The same features were recorded in tributary creeks, with all previous descriptions of these locations clearly failing to recognise the various sedimentary relationships. Distinct, thin, yellow clay horizons within the uppermost Maastrichtian mudstones are present in some sections. Geochemical analysis and radiometric dating have confirmed these as volcanic ashes, with extracted zircons giving a date of 65.95+0.04 Ma. These ash bands are located in Maastrichtian mudstones just below the K/Pg boundary and the recorded date is, within error, that of the K/Pg boundary. This identification of this latest Maastrichtian volcanic ash negates the suggestion of a pre-K/Pg boundary impact, a pre-extinction impact or multiple impacts. The benthic foraminiferal data generated indicates significant changes in palaeoecology of the benthic foraminifera across the K/Pg boundary. The benthic foraminifera do not experience a mass extinction, unlike the planktic foraminifera, which were significantly affected by the end-Maastrichtian extinction event. The benthic foraminferal assemblage appears to only experience transient, short-lived changes with pulses of agglutinated, elongate and large species in the early Paleocene. Mono-specific samples of Lenticulina rotulata have been analysed for stable isotopes and the data may indicate the presence cyclicity across the K/Pg boundary interval. In the earliest Paleocene significant negative δ18O excursions near the Pα/P1a and NP1/NP2 boundary represents a potential hyperthermal event that may be coeval with the DAN-C2 and Lower C29n events respectively recorded at Gubbio, and in the Atlantic Ocean. A sequence stratigraphy package is determined based on the micropaleontology and sedimentology in the Brazos River area. The latest Maastrichtian is marked by a sealevel rise immediately before the K/Pg boundary. Immediately after the K/Pg event, sealevel fell and is recorded as a change from mid to inner shelf. The condensed unit of the Middle Sandstone Bed (MSB) represents a Transgressive systems tract, with increasing diversity and abundance of benthic foraminifera to the top of the MSB, where maximum abundance and diversity is marks a Maximum Flooding Surface. The interval above indicates sea-level continuing to rise to a mid to outer-shelf setting. These sealevel changes are also recorded at the same stratigraphic level in Alabama, and at more distal K/Pg boundary sections (e.g., Denmark and Tunisia) suggesting that these sealevel changes are eustatically controlled.