A study is made of the geographical-temporal relationship between induced geoelectric fields and operational interference ("anomalies") experienced on power grids of the contiguous United States during the March 1989 magnetic superstorm. For this, we construct an hour-by-hour map sequence of peak 1-minute-resolution, horizontal-component geoelectric field amplitude, calculated by convolving maps of ground-level geomagnetic field variation, based on time series from magnetic observatories, with long-period impedance tensors, acquired during magnetotelluric surveys. These geoelectric amplitude maps and those of the locations of power-grid anomalies show that, during the March 1989 storm, anomalies were experienced when and where geoelectric amplitudes were high. Peak storm-time 1-minute-resolution geoelectric amplitude ranged from 21.66~V/km, at a survey site in Maine, to less than 0.02~V/km, at a site in Idaho (the ratio of the amplitudes is 1250). More generally, during the storm, high geoelectric amplitudes and numerous anomalies were realized across geographic areas with electrically resistive lithosphere in parts of the northern Midwest and the Northeast United States; low geoelectric amplitudes and few anomalies were realized across areas with electrically conductive lithosphere in other parts of the Midwest and much of the West. The geoelectric maps developed here show significantly more geographic granularity and greater range in amplitude than the benchmark maps developed for the North American Electric Reliability Corporation. No specific feature in ionospheric-current systems could be identified as being especially efficient at generating hazardous geoelectric fields, though this conclusion is limited by the sparsity of magnetic observatories in operation during March 1989.