IMPACTS OF HOT SPRING DISCHARGE ON THE RIO GRANDE IN BIG BEND NATIONAL PARK

The Rio Grande forms the southern boundary of Big Bend National Park (BBNP), and the International boundary between the United States and Mexico. The natural condition of the river has been degraded significantly due to a variety of factors including upstream diversion, upstream pollution, and drought. Of particular significance is the current low flow status of the river, a condition that has generally existed since approximately 1996. We have initiated a study to assess the contributions of thermal springs on the river system while the overall discharge is low. Long term discharge data available from the International Boundary and Water Commission (IBWC; provisional data) show that the discharge since 1997 is approximately 21% compared to a comparable period in the 1980’s. Several springs discharge into the river, primarily in the eastern portion of the BBNP reach and the Lower Canyons area downstream (east) from the park. Many of these are thermal springs that are fed by deep circulating meteoric water heated as a result of the local geothermal gradient (Henry, 1979). These springs are likely to contribute a significant portion of the overall flow. The combined effect of these springs on the total discharge of the Rio Grande can be seen in the IBWC data for Foster Ranch (located downstream from BBNP and the Lower Canyons), where discharge values are typically 30% higher than those at Johnson Ranch (located in the western portion of the park upstream of the springs). Our study is focused on one of the thermal springs in the eastern part of BBNP, located near the Gravel Pits campground. At this one spring we have seen very constant water conditions through the duration of 5 months of monitoring (T=40.6 oC, pH = 6.8, specific conductivity = 1358 uS/cm, HCO3 = 177 mg/L). The chemical conditions of the Rio Grande above the spring have been much more variable. For example, the specific conductivity has varied from a high of 2644 to a low of 1032 uS/cm. This variation is the result of the input of local precipitation and is correlative with discharge. In March of 2002, the spring provided 0.17 cms (6 cfs), or 12%, of the total river flow of 1.47 cms (52 cfs). The physical/chemical condition of the river downstream of the spring discharge area reflects the expected mixing of the river and the thermal water.