Digital elevation models (DEMs) are topographic landscape models that are easily created using geographic information system (GIS) software with elevation data available from the USGS. Once created the models can be altered to show how landscapes may have changed through time. In this case the DEM for the area of the central Connecticut River Valley in Massachusetts was tilted 0.889 m/km along a bearing of N 20.5° W (Koteff et al., 1993) to cancel the effect of post-glacial isostatic rebound that has tilted the Lake Hitchcock water plane.

As continental glacial ice retreated from New England, Glacial Lake Hitchcock filled the Connecticut River Valley from the drift dam at Rocky Hill Connecticut north to the vicinity of Lyme New Hampshire. Breaching of the drift dam led to draining of the lake south of the Holyoke Range but isostatically depressed lowlands north of the range coupled with large deltas blocking the valley south of the range allowed low-level lake stages to exist in this area.

The isostatically adjusted DEM shows that not all the major deltas in this area occur at the same elevation. The Sunderland delta is significantly higher than the Montague delta, suggesting that either these were formed under different lake stages or there are errors in the isostatic rebound correction. With lowering of lake level from the stable-phase stage the area north of the Holyoke Range remains flooded. Without isostatic rebound, the valley floor is essentially flat. Rebound is required to allow the Connecticut River to begin downcutting into the lake floor. Using the adjusted DEM it is easy to see how the configuration of Lake Hitchcock changed with lowering lake level.