81 
Figure VII-10. Example lower York River polyhaline segment YRKPH Dataflow sam¬ 
pling cruise track on August 25, 2003. 
Source: Virginia Institute of Marine Science—www2.vims.edu/vecos.. 
Water clarity criteria attainment can also be assessed in each segment’s shallow- 
water designated-use habitat through application of the CFD-based methodology 
described in Chapter 2 for each three-year assessment period. Exceedance is the 
cumulative frequency distribution of the portion of this zone that failed the K d - 
equivalent of the application depth specific water clarity criterion determined for that 
segment compared to a reference CFD curve. 
Naturally, environmental conditions will result in periodic exceedances of bay grass 
water clarity requirements; such exceedances are allowable for bay grass survival 
(U.S. EPA 2003a). Since allowable exceedances can be specific to salinity-based bay 
grass communities, biologically based reference curves are applied using measured 
water clarity exceedances established from existing bay grass beds for each salinity 
region using mid-channel water quality data (see Figures VI-1 and VI-2 in Chapter 
5). Figure VII-11 shows a preliminary example of a biological reference curve of 
water clarity exceedances based on shallow-water monitoring data for established 
bay grass beds in the polyhaline lower York River segment (YRKPH) during the 
2003 and 2004 growing seasons. This curve is plotted along with the previously 
published water clarity reference curve for mesohaline/polyhaline shallow-water bay 
grass designated-use habitats (U.S. EPA 2003a). 
chapter vii 
Shallow-water Monitoring and Application for Criteria Assessment 
