How Much Water is in the Pedernales? Occurrence of Flowing Water and Water Quality during Base Flow Conditions in the Pedernales River Basin
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The Pedernales River is in relatively good condition with respect to base flow conditions and base flow water quality. The Pedernales River and its tributaries traverse parts of eight counties in the Hill Country of Central Texas and are an important contributor of water to Lake Travis, the source of water for the City of Austin. Multiple aquifers contribute to base flow in the Pedernales River. Springs and streams originating in the Edwards and Glen Rose Aquifers appear to provide the majority of the main channel base flow in the western part of the Pedernales Basin. The Paleozoic and Trinity Aquifers contribute to base flow in the eastern basin area.
Changes in land cover from 2001 to 2011 indicate land cover did not significantly change over the ten year period, and the basin is generally scrubland and forest. However the amount of developed land increased in the Fredericksburg and Johnson City areas. Increasing impervious cover in developed areas may have implications for storm water quality. Due to the sandy nature of the Hensel (red sands) versus the carbonate characteristics of the majority of the other geologic units in the basin, areas underlain by Hensel are more buildable and amenable to agricultural and urban development and therefore more susceptible to water quality impacts.
Overall, the Pedernales River is a gaining river, meaning flow generally increases moving downstream, though there are losing reaches where surface water recharges the underlying aquifers. Common gaining and losing reaches were observed in the 1962 and 2016 gain/loss studies. Gaining and losing reaches are generally attributable to the underlying geology, though groundwater pumpage may be influencing Pedernales River base flow near the City of Fredericksburg. A significant gaining reach occurs between Johnson City and the confluence with Lake Travis. The occurrence of multiple droughts over the last decade and a half have caused a significant decrease in base flow of the river.
In general, water quality in the river under base flow conditions is good. While there have been changes in water quality, at least partly due to human impact, there have not been significant changes since a comparable study in the 1960s was performed. Analysis of several water chemistry parameters indicate water chemistry is primarily influenced by geology and land cover.
Now is the time to gain a more solid understanding of natural systems and the interconnectedness between surface and groundwater for water planning, wise water policy and the health of Hill Country springs, streams and rivers in the future.