A primer for watershed hydrology
By Gretchen Messer
William Henry Seward called Skaneateles Lake “The most beautiful body of water in the world.” It is also considered the second cleanest lake in the United States as measured by dissolved nitrogen, after Crater Lake in Oregon. And it is in our backyard.
As you may know, Skaneateles Lake is 16 miles long and 3/4 mile across at its widest point. Its deepest point is 315 feet, and the watershed covers 37,952 acres (Onondaga Soil and Water Conservation District). This ratio of land to water is very small in comparison to the watersheds of the other nearby Finger Lakes. Approximately 150 tributaries transport water and sediment directly to the Lake. And of course, the Lake receives water from precipitation, groundwater flow, and surface runoff. Skaneateles Lake naturally has very low levels of nutrients and high levels of dissolved oxygen. This combination of factors work to limit the scale of the biotic (living) community that the lake can support. It is also why the occurrence of blue green algae blooms was such a surprise.
Watersheds are comprised of headwater areas, drainage lands, and the water body. This includes the soils and minerals, plants and animals, and all human development. The natural processes of rainfall, runoff, groundwater recharge, sediment transport, and plant succession all operate to create a dynamic watershed system. It is important to view watershed as active and changing because we are part of it. Beyond the geology that created the basin, it is the hydrology that addresses the distribution, circulation, and behavior of the chemicals and physical actions of water movement in this system. The quality of the water is affected by numerous elements, primarily what the precipitation falls on and what it flows across.
The land cover of the Skaneateles Lake watershed is quite diverse. There are lawns and golf courses, roads and driveways, buildings and houses. There are also forests and farms, fields and wetlands. Each of these affects the water quality and how quickly the water reaches the lake. According to the Center for Watershed Protection, watersheds with greater than 10% impervious surface cover have degraded or impacted waters. Of the 37,952-acre watershed, currently 35% is considered open space, 40% is deemed agricultural, 24% is residential and 1% is commercial (Onondaga Soil and Water Conservation District). These land covers can’t be directly translated into impervious land amounts, but do provide a look at the watershed makeup.
Sediment is carried from each of these land covers. As a simple rule, the most densely vegetated areas release the least amount of sediment. Lawns and golf courses that are treated with fertilizers (primarily nitrogen for shoot growth, phosphorus for root growth, and potassium to help plants withstand natural stresses), pesticides and herbicides can release some of these nutrients if they are applied at rates and times when not needed by the vegetation. When released, they can be flushed to the lake with rainwater or irrigation systems.
Roads, driveways, buildings and houses are covered in dust – sediment. Some of this sediment is inert – just dirt from the land. Some is tainted by oils, tars, and other chemicals stemming from snow removal and automobile use (salts, oils and greases, antifreeze, rust, rubber, asbestos and more). These pollutants, too, end up in the lake.
As for forests, fields and wetlands, they supply organic material – waste from wildlife and decaying materials. These areas are important as they highlight the differences between infiltration and impervious surfaces. Vegetated areas allow precipitation to slow down and infiltrate into the soils, providing water for plants and recharging groundwater. Impervious lands turn stormwater into runoff where waters can quickly collect pollutants as they flow to the lake. Drainage ditches, while helpful and necessary in clearing water from roadways, also function to speed the flow of surface runoff to the lake.
So, watershed systems are quite comprehensive. The patterns and composition of the land cover as well as our actions directly influence the water quality. There are ways we can control our impacts, and we can begin now.
Sep 22, 2018
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