Abstract: A method for determining a volume fraction of water moving in a predetermined direction along a highly inclined conduit. The method includes measuring a fractional volume of water occupying the conduit at a plurality of locations along the conduit. An oxygen activation velocity of the water flowing in the conduit is determined at a plurality of locations along the conduit. A counting rate of a gamma ray detector used to measure oxygen activation is normalized with respect to the measured fractional volume of water. The normalized count rates of the gamma ray detector are characterized with respect to a relative velocity between the water and the detector. The step of characterizing is performed in portions of the conduit which are sloped so that gravity acts on the water along the predetermined direction. A fraction of the characterized counting rate represented by the oxygen activation counting rates measured along the conduit is determined.