A surface motion survey of Black Rapids Glacier, Alaska, USA

We describe a derivation of surface velocities and associated errors for Black Rapids Glacier, Alaska, U.S.A., using single-orbital-path synthetic aperture radar interferometry (InSAR). The technique described is adapted to small temperate glaciers with complex flow patterns. We also describe a motion anomaly, apparent in the InSAR phase signal, that persisted on Black Rapids Glacier for at least 78 days during winter 1991/92 and recurred in 1996. This anomaly is interpreted using a basal hydrology hypothesis in which a hydraulic head is maintained at the glacier bed at close to the overburden pressure. This permits a cumulative influx of 1.6 x 10(6) m(3) of water under the glacier, a sort of shallow subglacial lake, that migrates downstream at an average rate of 30 in d(-1) over 78 days. The motion anomaly is speculated to be an unsuccessful bid for surge initiation.