Vertical pulse sounding at 30, 60, 180, and 520 kHz at Quabbin Reservoir, MA, USA, reveals diurnal and seasonal variations in structure, reflectivity, and height of the lower ionosphere. A solid state wideband amplifier feeds 10 kW peak power into a ground-laid six-element antenna. A special digital sampling and integration scheme selects median values of amplitude and phase received at perpendicular loop antennas. Day and nighttime D-region model profiles are fitted to the observed data by matching group height and reflection coefficient. Emphasis is placed on the nighttime residual E-layer observed at 100 km with an electron density, under undisturbed conditions, around 1000/cc. Long and short term variability of this layer is considerable. It is often semi-transparent as indicated by simultaneous E- and F-region echoes, suggesting the presence of a valley above the nighttime E-layer. (Author)