In its natural state, the east watershed of Mirror Lake was quite large, about 20 hectares in area, which was nearly as large as the west watershed (about 24 hectares) and 2/3 the size of the northwest watershed (about 30 hectares). As a result, in its natural state, runoff from the east watershed must have been a major contributor of water to the lake.

Berm to capture runoff from I-93

However, as part of the construction of Interstate Highway I-93, a berm was constructed between the highway and Mirror Lake for the purpose of preventing highway runoff from entering the lake. The berm also diverted runoff from most of the east watershed of the lake uphill from the highway, reducing the size of the east watershed that contributes water to the lake from 20 hectares to 2.6 hectares. As a result of the diversion, a very small amount of surface water now enters Mirror Lake from its east watershed.