Measurements of riverbed hydraulic conductivity in a semi-arid lowland river system (Murray-Darling Basin, Australia)

Riverbed hydraulic conductivity (K-r) was measured along one river reach in four tributaries of the Murray-Darling Basin (MDB) in south-eastern Australia. Two techniques were trialled: in-river falling-head tests in high K-r sediments, and laboratory evaporation tests on intact riverbed cores for low K-r sediments. In-river falling-head tests were conducted using two types of permeameter: a steel-base permeameter or a stand-pipe permeameter. K-r was found to range from 10(-10) to 10(-3) m s(-1), corresponding to a range in riverbed sediment textures from clay to silty gravels, respectively. Although the within-reach variability in K-r was also large, in general the river reaches could be divided in two groups, those with a low K-r (<10(-8) m s(-1)) or a high K-r (>10(-5) m s(-1)). The low K-r reach (Billabong Creek) was a clay-lined bed, whereas the others had silty sand or silty gravel beds. Thus, regional-scale assessments of K-r in the MDB could be made using a stratified sampling process in which reaches would be first classified into low or high K-r classes, and then K-r measurements made in a subsample of low and high K-r reaches. This would be an improvement over the current practice whereby riverbed K-r is estimated either from regional soil maps or through the calibration of groundwater models.