IMPACT OF CHANNEL ENTRENCHMENT ON STREAM-AQUIFER INTERACTIONS AND THE RIPARIAN ZONE IN THE ARKANSAS RIVER VALLEY OF SOUTH-CENTRAL KANSAS

Anthropogenic alterations in hydrology and land use in the Arkansas River valley from southeastern Colorado to south-central Kansas have changed the river channel characteristics since the late 1800s. The changes include narrowing and deepening of the active channel, establishment of a riparian zone on the old broad channel, and die-off of woody vegetation (mainly phreatophytes) in that riparian zone. Johnson, Spray, and Dort (1994) describe these changes and their causes for the Arkansas River in southwest Kansas (Colorado-Kansas state line through Ford County). These changes have occurred in a delayed fashion in the river valley to the east in south-central Kansas (Edwards to Barton counties). The anthropogenic causes of these changes are regulation of the river flow by dams, alteration of runoff from land-use changes, and declining water tables from pumping. An increase in ground-water discharge occurs during channel deepening, leading to further water-level declines in the alluvial aquifer. Establishment of a riparian zone of phreatophytes over the prior broad channel further reduces water levels by increasing the evapotranspiration consumption of ground water. Direct recharge from precipitation that occurs rapidly in the un-vegetated zone of coarse river sediments in the entrenched channel (Cook et al., 2006) is less in the riparian zone, probably as a result of plant and surface litter interception, plant uptake, and the greater thickness of the unsaturated zone. An entrenched channel also decreases the surface area for ground-water recharge during high flow events that do not exceed bank full stage. Development of a riparian zone over the old broad channel appears to have occurred during the last several decades within the Edwards to Barton counties stretch of the Arkansas River. Ground-water declines from pumping have now become great enough along this stretch that a substantial portion of the native phreatophytes (mainly cottonwoods) are dying. Salt cedar (tamarisk), a non-native invasive species, spread across substantial areas of the Arkansas River channel in southwest Kansas during the last several decades. A concern is whether salt cedar stands will now increase in the riparian areas of south-central Kansas where native species are disappearing.