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The Sheepscot River Turbidity Pilot Study, - page 9 / 12

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As before, we find that this is enriched in comparison to the regional studies (15 and 10 ug/L for the Smith et al and EPA studies respectively).

TSS and turbidity values are very similar for a given sample site and date as demonstrated by correlation analysis (Table 4).  Again we see evidence that each storm is different (e.g. the correlation with TSS and turbidity is better for October than for May).  Turbidity and TP are closely related, while the relationship between turbidity and TN is not as good.  The turbidity tube measurements turn out to be a poor predictor of lab turbidity in NTU.  However, water clarity may be a more important variable in some ways for fish, since salmon are visually-oriented ambush predators.  Also, water clarity is easy to measure in the field for instant comparisons between sites.  Turbidity tubes are sometimes used to follow muddy water upstream or downstream to sediment sources.

Table 4.  A correlation matrix of different parameters by sample date.

Correlation Matrix

Date

R-square

TSS vs Turb

May

0.8094

TSS vs Turb

October

0.9686

Turb vs TP

June

0.9421

Turb vs TN

June

0.6618

Turb vs Clarity

June

0.4462

Conclusions:

During the 2006 field season, we used upstream and downstream sites to help us identify river reaches where there are sediment or nutrient sources.  We found some, but the sediment sources were different for different storms and different seasons.  No doubt, this is due in part to changes in land use within the watershed.  For instance, agricultural land may be ploughed in the spring, but have 100% cover during the growing season, and then be either fallow or have a cover crop in the fall.  With respect to Meadow Brook, the SVCA reports that this sub-watershed was heavily logged during 2006.  This could be the source of the observed turbidity in this stream.  Seasonal changes in hydrology are also important.  In the spring and fall, deciduous trees are bare and air temperatures are low, so that evapotranspiration is minimal.  Under these conditions, soils become easily waterlogged and generate more runoff.  In spite of this, our biggest sample day runoff event was a combination of storms in June (with a total of 4.85 inches of rain over six days).  Apparently the unusually wet June weather in 2006 caused our soils to become waterlogged in spite of summer evapotranspiration losses.  Our summertime turbidities were low, especially given the extremely high discharge.  This may suggest that the river flushes itself out in the spring and that storms later in the year entrain sediments from terrestrial “new sediment” sources.  

Given the changes in land use in the watershed (e.g., the logging around Meadow Brook), we would like to see more sampling of tributaries.  Maine’s Shoreland Zoning Act

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