Size Matters

The relative size of a watershed vs. the area of the lake it drains into has a large influence on water quality, independent of land use in the watershed. Lakes which are small relative to the area of their watersheds are subject to higher loading of nutrients per unit area and shorter detention times (more rapid flushing). As the watershed gets larger or the lake gets smaller, lake water quality becomes more and more a function of incoming stream water quality. Internal processes may not matter much when the watershed is delivering large amounts of water and associated contaminants such as sediment, bacteria, nitrogen and phosphorus.

On the other hand, a larger lake with a smaller watershed may be minimally affected by its watershed on a day to day or week to week basis, with seasonal influences or even annual influences being the shortest time frame for change identifiable by monitoring. For lakes with detention times of close to a year or more, internal processes may dominate water quality, such as loss of oxygen in deep water and release of phosphorus from affected sediment. The inputs from the watershed matter in the long run, but do not greatly affect daily to monthly conditions when the incoming water is such a small percentage of the lake volume.

There is not an official cutoff for small and large watershed to lake area ratios, but a generally accepted scale has ratios of 50:1 having very high watershed influence on a short term basis. In all cases land use matters, but in the range of ratios of 20-30:1 that influence becomes very important. A watershed that is 20 times the size of the lake it drains into may have limited negative impacts if all forested, while the same ratio for a largely urbanized watershed may greatly impact water quality in the lake after storms.

It is therefore recommended that lake studies include a careful analysis of watershed to lake area ratio and land use within that watershed. Simple but fairly reliable models can be used to predict water quality based on these and a few other easily obtained watershed features, and represent an economical head start to understanding your lake and its management needs. Watershed and land use data can be readily obtained online for most areas, with a host of GIS-based programs available for use by those with minimal cartography skills. Try these out! It is actually fun as well as very educational. Do you know what flows to your lake?

Watershed size and land use mapping is now pretty easy from the comfort of your home.