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Long-Term Sedimentation Rates and Patterns of Alluvial Ridge Basins (Esteros) in the Rio Grande Delta, US, and Mexico

Session: Geomorphology and Surface Processes Across the Solar System

Presenting Author:

Kynan Rutan

Authors:

Rutan, Kynan¹, Dong, Tian Yang², Nabi, Waqid³, Gonzalez, Juan Luis⁴, Kenney, William⁵

(1) University of Texas Rio Grande Valley, Edinburg, Texas, USA, (2) University of Texas Rio Grande Valley, Edinburg, TX, USA, (3) University of Texas Rio Grande Valley, Edinburg, TX, USA, (4) University of Texas Rio Grande Valley, Edinburg, TX, USA, (5) University of Florida, Gainesville, Florida, USA,

Abstract:

A deltaic plane's topographic variability is impacted by the alluvial ridges of a river system, which form on the edges of a channel during active flow. After an avulsion event, abandoned channels retain their alluvial ridges for an extended period (>10² years). Multiple generations of abandoned channels create a landscape of alluvial ridges bounding low-relief basins, called alluvial ridge (AR) basins, within the deltaic plain. In the Rio Grande Delta (RGD), AR basins often contain tidal lakes known as Esteros, receiving water and sediment from the Gulf of Mexico via connecting channels. Although work has been done to understand the biological aspects of these lakes, little is known about the sediment dynamics, hindering a holistic understanding of deltaic land-building processes. To better understand these dynamics, we examined long-term sedimentation rates and patterns within a series of AR basins, specifically the Bahia Grande (BG) complex. Twenty-three sediment cores, ranging from 0.35 to 1.3m below the lakebed, were taken in four Esteros of the BG complex via peat-, push-, hammer-, and vibra- coring devices and sub-sampled at 2−5 cm intervals. Sub-samples were processed for grain size, organic matter content, and Pb-210 dating to understand long-term spatiotemporal sedimentation trends. Additionally, four cores over 1m deep are analyzed using XRF to determine their elemental composition and understand the geochemical changes in the BG complex. Results show alternating coarse and fine sediment layers with evaporated mineral layers intermixed, and a distinct oxidized clay layer starting at a depth of ~20–40 cm across all four Esteros. We hypothesize that sedimentation is controlled by tidal hydrodynamics, human intervention, and hurricane events. Specifically, coarser sediment layers are thought to reflect extreme tidal events, as explained by the threshold theory of sediment transport, allowing for correlation with recorded storm events. Evaporated mineral layers are believed to result from the natural wet and dry cycles that occur within the BG complex. Furthermore, it is hypothesized that the oxidized clay layer resulted from aeolian sediment transport of the dry lakebed during a “dust bowl” period, when tidal flow was blocked from entering the system, with drier conditions present in the northern basin. The study’s results will improve understanding of land-building processes in the RGD, informing engineering and environmental management as the area continues to develop rapidly.