Development Alters the Lake’s Hydrology

The pre-colonial natural landscape for the Berkshires was almost completely forested, with open lakes and marshes creating breaks in the tree canopy. Native Americans moved across the region, following the seasons and clearing small areas for temporary villages. The open patches they left behind transitioned into areas dominated by grasses and ferns before returning once more to forest.

When rain falls to the ground in a naturally forest area, as much as 50% of it percolates into the ground, 40% re-enters the atmosphere and 10% flows overland as surface runoff. The tree canopy intercepts the raindrops, while the vegetation and leaf litter on the forest floor acts like a sponge, soaking up the water. When the rainwater percolates into the soil, the vegetation, soil and microorganisms absorb and filter out most of the pollutants that are traveling in surface runoff.

When we remove the forest for development we disturb the soil and leaf litter, and build impervious surface areas such as roads, building rooftops, driveways and patios. In addition, lawns act as semi-impervious surface areas because we graded and compacted the soil during construction of our homes and we reinforce the compaction each time we run our lawnmowers over it. As a result of our actions, percolation of rain and snow is reduced and surface runoff is increased — and the faster water flows across the land the greater its capacity to carry sediment and pollution. Creating as little as 10-20% impervious surface area on a residential property can double the rate of runoff from the site.

Surface runoff, resulting from development during rain storms and snowmelt, is the single largest contributor to water quality degradation in Massachusetts. Increased surface runoff is a problem in that it alters the hydrologic cycle within the watershed of the lake, causing higher peak storm flows in the tributaries and leading to increased sediment deposition in the lake. It is estimated that 80-90% of phosphorus reaches waterways adhered to sediment particles, loosened from the soil and traveling in runoff.

Sediment and Phosphorus are Linked

Phosphorus is a nutrient that is normally found in the natural environment, including lakes. Phosphorus is typically described as the limiting nutrient for fresh water bodies in the Northeast, including the Berkshires. This means that the amount of phosphorus within the lake directly affects the productivity of the lake and the ability for plants and algae to flourish, just like adding fertilizer to a lawn or garden. The long-term phosphorus budget for the lake indicates that the major phosphorus inputs are these: 56% is from land clearing and development and 41% is from in-lake sediments (Diagnostic/Feasibility Study for Stockbridge Bowl, 1991).

The Bad News is that runoff from cleared and developed land contains much higher amounts of phosphorus than runoff from undeveloped forests. One recent study in Maine found that even careful development of forest into two-acre house lots caused a 2-10 fold increase in phosphorus concentrations in stormwater runoff. Residential properties can contribute roughly .5 – 1 pounds of phosphorus per acre to a receiving waterway each year! Less than 2 ounces of fertilizer washed into the lake can grow more than 2 pounds of algae. The Good News is that if we all work together and make small adjustments on our properties, we can reduce the amount of surface runoff and phosphorus that enters our lake.