A better title to this discussion might be “How we contaminate our lakes”. Contaminate according to Merriam-Webster is to “soil, stain, corrupt, or infect by contact or association” or “to make unfit for use by the introduction of unwholesome or undesirable elements”. Pollute on the other hand (according to Merriam-Webster) is “to make ceremonially or morally impure” or “to make physically impure or unclean”. In the scientific literature, “contaminate” is the preferred word over “pollute”. It’s a semantic argument, but none the less, a better choice.
In any case, the subject is how we mess up our lakes, and I will include reservoirs in this discussion, too. Reservoirs are impounded (dammed) streams or rivers that flood river valleys in mountain terrain or flood plains in alluvial terrain. Reservoirs along with lakes are important sources of drinking, industrial, and agricultural (irrigation) water supply, hydroelectric power, and support fisheries and wildlife habitat, recreation, and navigation. Most inland urban areas are located along a river or lake and most rivers have been engineered one way or another to control flow.
Lakes and reservoirs are the sink in which much of everything in its catchment (the landscape that drains into a lake or reservoir) is transported into and is stored, metabolized or transformed, and/or exported out. Any activity on the landscape, other than what was (is) natural, provides opportunity to contaminate our lakes and reservoirs. Most common, but not always considered important, is the mere physical alteration of the landscape changing the way water flows off the landscape into the streams and rivers and ultimately into the lake or reservoir. Deforestation into pasture or row-crop agriculture, urban, industrial, low- and/or high-density residential, changes the precipitation/runoff relations, the energy required to pick up soil and sediments and deliver them to the stream channel and ultimately to the lake or reservoir. Clearing vegetation to the stream bank, eliminating the riparian zone (the interface between land and the stream channel), weakens stream bank stabilization and promotes stream bank failure or collapse, delivering sediments to the lake or reservoir. These sediments, in addition to sediments delivered from erosion sources (construction sites, agriculture landscapes, and dirt roads), cause lakes and reservoirs to fill up more quickly than would naturally occur. These sediments carry nutrients, metals, organic material, and pathogens with them. Suspended sediments in the water column of lakes and reservoirs block light and therefore energy needed and used by photosynthetic micro-organisms to produce biomass, the basis of the food chain in lake and reservoir environments.
Once the landscape in a lake or reservoir catchment has been changed, altered, or modified to provide opportunity for agriculture, urban, industrial, low- and/or high-density residential development, materials (fertilizers, pesticides, industrial materials, pharmaceuticals, and other consumable products) imported into the catchment to support these activities have the potential to enter the streams and rivers and ultimately into the receiving lake or reservoir. Two main mechanisms of contaminant transport into streams from the landscape are “point” and “non point”. Point sources are easily identified as waste-water treatment plant discharges, industrial discharges, and such. These are generally regulated by federal and state regulatory agencies and can be managed accordingly. Non-point sources are not as easily regulated and managed. Their sources are usually diffuse, spread out across the landscape, and difficult to identify and quantify. Sources include agriculture and urban runoff. Contributions from mine wastes and landfill leachates and such, can also be considered non-point source contributions. Contaminated groundwater can enter the stream and ultimately into lakes and reservoirs through base flow contributions. And there are other sources of contaminants, as well.
How do we pollute or contaminate our lakes and reservoirs? By definition (see above), to contaminant is to “soil, stain, corrupt, or infect by contact or association”. Simply, by changing, altering, modifying the landscape within the catchment of a lake or reservoir, we provide opportunity to contaminate our lakes and reservoirs. Materials are imported into the catchment that would not otherwise be there; landscapes are altered to meet the needs of agriculture, industrial, urban, and residential development. Is this bad? Not always. It is a matter of sustainability, meeting the needs of civilization without degrading the environment.