[en] A study of ¹³⁷Cs in rivers implies a knowledge of its adsorption behaviour towards particulate matter and sediments and the possibility of low-level counting of water and solids. This text focuses on the low level counting of ¹³⁷Cs. The use of flat-ended Ge(Li) detectors is compared to that of an anticoincidence array after preconcentration of the cesium ions into a≤0.5 ml solid aliquot. The eventual improvement of the accuracy by applying 'moving average' and 'smoothing' procedures are considered. (author) 10 refs.; 1 fig.; 4 tabs