ER I INVESTIGATION OF AN ANTECEDENT FLOODPLAIN CHANNEL-BELT

摘要

Hydro logic models of floodpiain architecture often assume that the stratigraphic units and hydraulic head between monitoring wells are laterally continuous.A recent application of this assumption to the Brazos River Alluvial Aquifer(BR AA)of Texas could not satisfactorily conform the model to hydraulic head data observed at monitoring welis.These data showed that,at a few hundred meters away from the Brazos,the hydraulic head abruptly rose ~4 m above the general trend closer to the river and,over 8 months of monitoring,the discontinuity did not level out.The researchers hypothesized that antecedent,buried,clay-lined channels of the Brazos had incised down to the underlying Eocene bedrock,isolating the modern river course from much of the alluvial aquifer,and created what might be termed”a highly compartmentalized aquifer”.Using electrical resistivity tomography,we have mapped the electrical resistivity on the west bank of the Brazos River.We imaged a putative buried sand channel and tracked it for-750 m parallel to the Brazos.Near the northern end,the putative channel may be fed by a surface tributary to the Brazos,and near the southern end.it appears to abruptly turn east and discharge into the Brazos at the location of a natural spring.Our results suggest that a discrete groundwater flow path through a buried”tube”of clay-lined sand divides the bank storage of the Brazos from the inner section(s)of the alluvial floodpiain aquifer.These results are consistent with the concept that alluvial floodpiain aquifers are not laterally continuous due to the presence of antecedent channel-belts that separate compartments of groundwater from the meandering,avulsing river.These channel-belts control the interaction between surface river water and groundwater-especially with regard to whether these channels are incised down to bedrock-and thereby also control the base flow input into rivers.This research has implications for streamflow generation and floodpiain architecture in general.