Powell Lake contains a deep layer of relic seawater separated from the ocean since the last ice age. Permanently stratified and geothermally heated from below, this deep layer is an isolated geophysical domain suitable for studying double-diffusive convection. High-resolution CTD and microstructure measurements show several double-diffusive staircases (R-rho = 1.6 to 6) in the deep water, separated vertically by smooth high-gradient regions with much larger density ratios. The lowest staircase contains steps that are laterally coherent on the basin scale and have a well-defined vertical structure. On average, temperature steps in this staircase are 4 mK, salinity steps are 2 mg kg(-1), and mixed layer heights are 70 cm. The CTD is capable of measuring bulk characteristics of the staircase in both temperature and salinity. Microstructure measurements are limited to temperature alone, but resolve the maximum temperature gradients in the center of selected laminar interfaces. Two different algorithms for characterizing the staircase are compared. Consistent estimates of the steady-state heat flux (27 mW m(-2)) are obtained from measurements above and below the staircase, as well as from microstructure measurements in the center of smooth interfaces. Estimates obtained from bulk interface gradients underestimate the steady-state flux by nearly a factor of 2. The mean flux calculated using a standard 4/3 flux law parameterization agrees well with the independent estimates, but inconsistencies between the parameterization and the observations remain. These inconsistencies are examined by comparing the underlying scaling relationship to the measurements.