Category Archives: Watershed Management

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.

Low Impact Development

Development and property management techniques that minimize water pollution impacts off-site are called Low Impact Development, or LID. There is some very clever engineering involved in some cases, but for the most part this is not rocket science. Sources on the property should be minimized, but recognizing limits on residential properties, the vehicle for off-site transport, runoff, is restricted. The idea is to limit impervious surfaces and collect as much runoff as possible for infiltration or detention on the property. The focus is on actions at the individual property level, rather than some larger downstream facility to hold greater quantities of runoff long enough to allow it to be purified by natural or engineered means.

Typical techniques include bioretention (rain gardens), porous pavement, grass swales, and green roofs. These techniques can work in almost any climate; having a cold winter is not really a deterrent. Sizing is important, but the most critical limitation tends to be soil type. Turning runoff into ground water provides excellent treatment and has not resulted in extensive ground water contamination, although each case must be considered individually. However, not all soils are conducive to receiving as much runoff as can be derived from roofs, driveways and packed lawn areas. Engineering in such cases is most critical and may be challenging. Where runoff cannot be percolated, it may be detained and purified by various means before being released to a stream or lake.

There is a fair amount of literature out there that explains the techniques and reviews results, but one does have to do a lot of reading to get up to speed. One accessible reference that is helpful and has an extensive reference list is Ahiablame, L.M., Engel, B.A. & Chaubey, I. Water Air Soil Pollut (2012) 223: 4253. doi:10.1007/s11270-012-1189-2. Another publication, free from the USEPA, is Reducing Stormwater Costs Through Low Impact Development Strategies and Practices. The key limitation to wider application has been insufficient documentation of results, which is a difficult problem. One can demonstrate success at the individual property level, but showing positive impacts on an entire lake ecosystem is challenging, since the extent of application has to be very high. Another troublesome aspect is that removal of phosphorus, probably the most important nutrient entering our lakes, is not especially high; one cannot completely counter the impact of development on a lake with LID. The good news is that there is very little downside; anything homeowners can do to limit contaminated runoff from leaving the home site will benefit the lake, and application of LID techniques is less expensive than alternative measures.

Reducing Stormwater Costs through Low Impact Development

Being Lake Smart

The best watershed management is applied at the source, but this means getting property owners to actively participate. Watershed management is a challenge, but there are programs to help. One such program is LakeSmart, a program of the Maine Lakes Society (MLS). LakeSmart educates, assists, and recognizes property owners who maintain their home sites in ways that manage storm water and waste water to minimize impacts on lakes. The program was created in 2004 by the Maine Department of Environmental Protection (DEP), expanded in 2009 by a partnership between the Society and DEP, and is now fully privatized under the Maine Lakes Society. The MLS presents its distinctive blue and white signs to homeowners who meet program criteria, and is approaching its 5-year goal of 60 lake association participants by 2018.

LakeSmart awards were presented to over 80 homeowners during the summer of 2015.   Posted at the lakeside and driveway entrances of a property, the distinctive blue and white sign identifies the owner as a person who cares enough to take action to protect the lake. Properties that display the sign show others what lake-friendly living looks like, arouse interest, and motivate similar behavior by other community members. The MLS model for running LakeSmart is cost-effective, leveraging the power, interest and commitment of lake association members to speed the program’s spread.

Many homeowners grew up with suburban landscaping and are accustomed to its tidy lawns and open space. But suburban lawns, with big driveways and wide paths, are deadly for our lakes. LakeSmart landscaping provides a healthy alternative that mimics nature’s rich mosaic of plants, shrubs, winding paths, and shady trees. It looks great, enhances privacy, and works hard to protect property values, wildlife habitat, water quality, recreational opportunities and the vitality of local economies. It looks even better when you understand how important it is to minimize nutrient inputs to lakes. It may be hard to believe that one person’s expansive lawn or eroding camp road could be a threat to something as large and enduring as a lake, but when added to a shoreline full of similar sites, it can have a very real impact, especially over multiple years. All storm water that gets into a lake carries nutrients. Over time, the cumulative impact can be thousands of pounds of pollutants. The result, “death by a thousand cuts,” leads to algae blooms, fish kills, and the loss of water clarity and spawning habitat. One tiny rivulet from one rainstorm may not seem like much, but when multiplied across a lake watershed and added up over decades, eroded soil can turn a lake into a smelly, pea green mess.

This is a program that can be applied anywhere. It is certainly easier to build a lake-friendly property from the start, but retrofitting and minimizing impacts is not really that hard in most cases. Check out the MLS LakeSmart program at http://mainelakessociety.org/lakesmart-2/ and contact Maggie Shannon at the Maine Lakes Society at msshannon@mainelakessociety.org  for more information.

Limits to Best Management Practices

A recent post discussed the role of watershed management in protection vs. restoration of lakes. The reason why watershed management cannot be a mainstay of lake restoration is not obvious to everyone, and here we explore the limits to best management practices in the watershed from the perspective of lake impacts.

Best management practices (BMPs) are procedural or structural techniques used to limit the delivery of contaminants from land to water and eventually the lake. BMPs may restrict what land uses or activities can occur, preventing generation of contaminant loads, or BMPs may focus on trapping contaminants on the way to the lake. “Best” does not necessarily mean adequate or effective, but it is often assumed that if all appropriate practices are applied, the lake will be protected. This is rarely true.

A well designed detention facility.

The USEPA has accumulated a huge database on the actual results from various management methods, with a focus on storm water BMPs, as non-point source inputs from developed or agricultural land are by far the biggest input sources these days. The overall average phosphorus reduction that is achieved by individual BMPs is about 50%. This is not the average of all possible projects, but those where the technique was properly applied and monitored; inadequate design, sizing or construction could provide less benefit. A few techniques approach the 90% mark for phosphorus removal, most notably infiltration into appropriate soils and inactivation and filtration, but these are rarely applicable on a watershed-wide basis.

Leaching basin.

Development and agriculture increase phosphorus loading by an order of magnitude or more in the absence of BMPs, which then reduce those loads by some percentage, averaging 50% on average where properly applied and less in many cases where implementation is incomplete or absent. But if we assume that all developed or agricultural uses are addressed by BMPs that yield a 50% reduction in phosphorus from the tenfold or greater increase expected without BMPs, we have a five-fold increase in loading. Even if we could achieve a 90% reduction, that still represents a doubling of loading from pre-development, pre-agricultural conditions. Human use of land is a losing equation for lakes.

This doesn’t mean that any development or agriculture will doom your lake. Where the human activity occurs and how it is managed matters a lot, and the overall percentage of the watershed in human uses is very important. Most practitioners agree that serious problems can be avoided up to about 25% of the watershed in developed or agricultural uses with judicious BMP application. However, it is simply not reasonable to assume that incoming water quality will be acceptable in urban areas (typically >75% developed) or farm country. The size of the watershed relative to the lake and the depth of the lake will matter too, so simple thresholds will not likely be reliable in all cases. Yet it is clear that where human activities dominate the landscape, application of BMPs will not be able to keep up with the generated loads.

Protection vs. Restoration

There has been a lot of discussion at the annual NALMS symposium over the last 5 years about the value of watershed management. Much of this discussion was a reaction to federal and state directives to focus on watershed management to solve lake problems, as we know from many years of experience that some problems cannot be solved with watershed management (e.g., internal phosphorus loading, excessive plant density) and watershed management has proven very difficult over large areas under multiple jurisdictions. In articles in LakeLine and presentations at NALMS meetings a few people have made statements that have been interpreted as opposing watershed management, but these statement are being misinterpreted in the context of protection vs. restoration.

First of all, there is very little lake restoration going on. We often rehabilitate lakes, altering them to meet use goals, but that is not the same thing as restoration. For lakes created by erecting a dam, true restoration would mean removing the dam and eliminating the lake; this is not usually what lake managers have in mind when they use the word “restoration”. But whether we call it restoration or rehabilitation, reducing nutrient levels, algae blooms and nuisance vascular plant growths is rarely achieved by watershed management once the lake gets to the point of supporting such growths. Once in a while we find the “smoking gun” and can implement a focused watershed management plan, but usually it is a slow, incremental process that very rarely moves the lake adequately in the right direction. Consequently, in-lake efforts are often needed to meet use goals, including phosphorus inactivation, dredging, herbicides, harvesting and other commonly applied methods.

Protection, however, is another matter. If a lake is in a desirable state, it is wrong to assume it will always be that way. Watershed influences can be gradual or catastrophic, but their presence is undeniable. If the watershed is large enough (>10:1 ratio with lake area is a commonly cited threshold), inputs over many years will eventually change the character of the lake, and water quality issues become more probable when the watershed is >50 times the area of the lake, even with no human activities in the lake. However, human activities greatly accelerate loading of sediment, nutrients and a variety of other contaminants. Thresholds as low as 6% development in the watershed have been cited as resulting in measurable changes in water quality, and at development levels on the order of 25-30% it is rare not to see deterioration of water quality. Watershed management is therefore essential to protecting lakes, but the potential to adequately protect the lake declines as the percentage of development or agriculture increases.

Developed vs. Forested Land.

Watershed management is a logical component of any lake management plan. It is wrong to hold an invasive plant control project hostage until the applicant produces a watershed management plan, but it is entirely reasonable to expect holistic lake management programs to incorporate watershed management. If protection of desirable features is the goal, watershed management is a must. If rehabilitation of degraded conditions is needed, watershed management is not likely to be the whole answer. Keep protection and restoration elements of any plan separate when discussing lake management to avoid controversy over the role of watershed management.

Agriculture by a Lake.

Climate change impacts quantitatively assessed

Some impacts of climate change have known for years, but others are still surfacing. About a decade ago a representative of the USGS in Maine presented data at the NECNALMS conference for the date that ice cover broke up for multiple lakes in Maine. While there was variability typical of systems influenced by climate change, it was very clear that the date the ice was going out was getting earlier over decades. In about a 60 year period the average ice out date for the 2000s was about 2-3 weeks earlier than it had been in the 1950s. This should have been impressive enough in its own right, but apparently few other than ice fisherman really took notice.

Now we have another interesting measure that might be scarier. Oxygen consumption is an important feature in lakes, causing oxygen to become depleted (called anoxia) in many lakes deep enough to stratify, at which point oxygen can’t rapidly move downward from upper waters and decomposition gradually removes  oxygen from the bottom up to the boundary point, called the thermocline. Fish like trout that need cold water (<21 oC) but high oxygen (>5 mg/L) can get “squeezed”, faced with water too warm above and water with too little oxygen below. There may be no “trout water” during late summer, causing mortality. Further, loss of oxygen in deep water can allow phosphorus bound to iron to be released into the water column where it can support algae blooms, most often cyanobacteria that are favored by this type of release. In shallow water, high oxygen demand is indicative of elevated decomposition, and while complete loss of oxygen in water <15 ft deep is rare, that decomposition releases phosphorus that can fuel algae blooms. So oxygen consumption matters a lot to lake condition.

Data from Long Pond in Brewster and Harwich on Cape Cod, collected by the Natural Resources Department as part of a very useful water quality monitoring program, were plotted in an effort to understand the variation in oxygen consumption observed over time. What was found was that a relatively small difference in temperature, brought on by warmer spring air temperatures, resulted in a major increases in the rate of oxygen consumption, or oxygen demand (see figure). Oxygen demand below about 550 mg/m2/day is considered unlikely to cause severe anoxia, while values higher than 1000 mg/m2/day usually cause most of the bottom water layer to become anoxic in August and values greater than 2000 mg/m2/day will typically cause anoxia in July. Long Pond has an oxygen depletion problem, and it worsens appreciably with increasing temperatures in the deepest water.

Using a statistical technique called regression, the portion of the variability attributable to any tested factor can be evaluated. For Long Pond, and probably many other lakes, change in temperature explains much of the variation in oxygen demand (62% for Long Pond, a high percentage for a single factor). And this doesn’t require a large change either; the range of oxygen demand in Long Pond more than doubles for a temperature increase of only 3 Co (5.4 Fo)! The influence of the current direction of climate change is pushing our lakes toward a higher metabolism, almost like they have a fever, and the implications for all users, human or otherwise, are not good.

Oxygen demand as a function of temperature in Long Pond, MA.

 

2016 Drought Highlights: Role of watersheds in lake condition

Looking down view tube at Secchi disk

Precipitation and flows were well below normal in spring and summer of 2016. For example, at Morses Pond in Wellesley, MA, there was no winter snow pack to speak of and precipitation in May through August was about half of the average for the previous decade (7.2 vs 14.2 inches). The situation was similar all over New England, and while evaporation exceeded precipitation during summer and caused low water levels, the reduction of nutrient inputs also resulted in high (sometimes record) water clarity. Blooms of algae were less common in lakes that are tightly linked to watershed inputs on a seasonal basis, which includes most impoundments on streams and river and other lakes with watersheds more than about 20 times the area of the lake. Phosphorus and chlorophyll-a concentrations were lower than average for a respective 72 and 80% of lakes surveyed by LEA in Maine, leading to Secchi transparency values higher than average for 72% of surveyed lakes. Unless internal recycling is the dominant source of phosphorus to a lake, reduced precipitation translates into less runoff, lower nutrient inputs, and higher water clarity.

The importance of a watershed to lake condition is clearly demonstrated, but that importance is mediated through two key processes: weather pattern and land management. In 2016 the weather did a lot of what we strive to do with land management, minimizing the transport of nutrients and other contaminants to lakes. We can’t control the weather, and having less water entering our lakes has its downsides (e.g., lower water levels, more impact from rooted plants), but the importance of watershed management to minimize nutrient inputs when the weather is not cooperating is underscored. If we can’t put a dome over our watershed and only open the roof when we want the water, we have to manage the watershed to limit inputs to the lake.

But what is the potential for watershed management to provide the benefits observed in 2016 as a result of low precipitation? The better than average conditions were associated with precipitation about 50% below normal. Nutrient loading is not necessarily proportional to water inputs, and we would expect disproportionately more loading with larger storms, but it seems reasonable to assume that we would need at least a 50% reduction of loading through watershed management to reap the same benefits provided by the 2016 weather pattern. Based on years of evaluation by the USEPA, phosphorus removal by best management practices rarely averages more than 50%, although well designed infiltration facilities can achieve 90% reductions. However, not all watershed soils are suitable for infiltration systems, so what all this means functionally is that we will be hard pressed to provide the level of watershed management necessary to maintain the conditions we observed in 2016.

We can view 2016 as setting the bar for potential lake condition with regard to nutrients, algae and water clarity. Low precipitation limited inputs, and while there were some negative effects of having less water, the water quality was about as high as could be expected in New England lakes. If your lake was not appreciably better than in other recent years, internal loading sources were most likely dominant or there is another source (e.g., direct discharge or extensive storm water piping that limits load reduction on the way to the lake) that requires attention. Yet for those lakes that did exhibit better than average conditions in 2016, maintaining those conditions by watershed management will require superior effort, as the practical limits to best management practices will necessitate application all over the watershed to achieve the level of loading reduction experienced in 2016.