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Tributary Monitoring
The Squam Lakes Water and Nutrient Budget Study
By Jeffrey Schloss, UNH Water Resources Specialist
As population expansion occurs in the Squam Lakes region and the resulting pressures from development and recreational use ensues, there is a growing concern over the degradation of lake water quality. Of primary concern are the impacts of cultural eutrophication; increased nutrient loading resulting in accelerated plant growth (submerged vascular plants and algae blooms) within the lakes. Nutrients can come from many sources and include surface runoff resulting from precipitation upon the natural and developed areas of the lake's drainage basin, commonly referred to as a watershed. Additional nutrients are transported into the lake through stream inflow, groundwater, septic system effluent and even from precipitation and dry fallout (dust particles). Of the two nutrients most important to the growth of aquatic plants, nitrogen and phosphorus, it is generally observed that phosphorus is the more limiting to plant growth in lakes, and therefore the more important to monitor and control. Phosphorus is generally present in lower concentrations, and its sources arise primarily through human related activity in a watershed. The best way to understand where the major sources of phosphorus are coming from is to conduct a watershed "nutrient budget" study.
In the late fall of 1998 UNH researchers surveyed the Squam Lakes Watershed and installed staff gauges (large ceramic coated steel rulers that measure bank height of a stream) at the major tributaries. Culverts that drained additional watershed areas and collected road runoff during storms were also inventoried for sampling. Between the spring of 1999 and July of 2000 UNH Center for Freshwater Biology scientists and students with the help of 18 resident volunteers and additional SLA staff were able to collect over 775 total phosphorus (TP) samples and take over 1000 stream staff gauge and culvert inflow measurements at over 35 different sites.
(Click here for a map of the sites)
This allowed coverage of low, normal ("base") and storm event flow conditions of the contributing streams and runoff areas through almost two years. By multiplying the TP concentrations by the volume of water flowing into the lake we get a measure of the actual TP load impacting the lake.
It's all a matter of "balance"!
Before you can get at the nutrient budget of a lake watershed you have to balance the water input and output. Besides stream flow and overland storm runoff, water enters a lake through direct precipitation upon the lake surface and through groundwater influx. Weather station data from Meredith and Plymouth allowed for precipitation accounting and analysis of the base flow from the major Squam tributaries allowed us to estimate groundwater influx. So how did we do? Luckily we had almost continuous coverage of lake outflow from an automated flow gauge at the Ashland dam and lake height from the RT. 3 bridge water level logger to figure out lake volume changes. When the computers stopped whizzing and whining we were able to balance the water inflow and outflow to within 2 percent! Considering that the Harvard University study done in the 1970s had a water budget error of as much as plus or minus 40% we did pretty well here. Kudos to the SLA!
The water/nutrient budget was calculated over a twelve month period from July 1999 - June 2000 Figure 2 displays the water budget results. Surface water sources dominated as major streams contributed 46% and seasonal streams combined with surface runoff contributed and additional 20% of the water. Precipitation contributed 31 % which is typical of our large NH lake watersheds where the watershed area is not extensive relative to lake area.
Groundwater influx was about 3 percent. Breaking it down by tributary, Owl Brook (which has been shown to influence the western end of Little Squam Lake) contributed 26.4% of the stream flow volume, while Smith Brook contributed 10.6% of the stream flow volume. Two other tributaries, the outlets from Barville Pond and White Oak Pond contributed 9.8% and 8.2% of the streamflow volume, respectively. The 28 additional streams monitored during this study contributed the remaining 45% of the Squam Lake streamflow volume, with none alone accounting for more than 3.8%. Like most New Hampshire lakes, the greatest volume of water entered the Squam Lakes during the period of spring melt in March, April and May. A large storm in September contributed a greater than normal percentage of yearly water input (12%) during that month.
The dominant source of phosphorus entering the Squam Lakes is from streamflow (47.7%), diffuse runoff (21.2%) and septic systems (21.4%). Contributions from atmospheric sources, wetfall and dryfall, contributed significantly less (9.7%) phosphorus. Phosphorus entering the Squam Lakes through streamflow closely mirrored the water inflow from the two larger tributaries with the Owl Brook inlet contributing 29.3% of the phosphorus load followed by Smith Brook (7.5%), while Barville Pond and White Oak tributary contributed 3.2% and 6.1% of the loads, respectively.
Except for the two Livermore Cove tributaries which accounted for 6.0% and 7.9% of phosphorus loading, the 26 smaller streams collectively contributed the remaining 40.0% phosphorus with no individual stream accounting for more than 3.8% of the loading.
What did we learn?
As the majority of the TP coming into the Squam Lakes results from water runoff throughout the watershed, thus, what we do on the watershed landscape is critical to the health of the lakes. Minimizing our impacts (see the SLA pamphlet 50 Ways to Save Squam Lake) will insure the best water quality. The overall phosphorus load to the Squam Lakes was low and characteristic of a relatively pristine, mostly forested, New Hampshire watershed. However, localized problems did exist when we compare relative water inflow to relative TP contribution, and TP contribution per subwatershed area. We will be investigating those critical areas further. Septic system contributions were also found to be significant and this study most likely under-calculates their true impact. Septic systems also contribute the greatest summer season loading compared to the other sources. This stresses the need to keep these systems maintained properly. More detailed explanations and results of this study are outlined a report to be provided to the SLA.
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