K-10 
uncontaminated degraded samples were found with excessive abundance/biomass 
and insufficient abundance/biomass, respectively. Insufficient sample size in Severn 
Creek (MOBPHe), Ware River (MOBPHf), and East River (MOBPHh), precluded 
reliable assessments of degradation within these segments. 
In summary, contaminants are likely to be substantial contributors to benthic 
community degradation in the York River, particularly in the lower Mattaponi River 
(MPNOHa) and the middle York River (YRKMHa). Contamination sources of stress 
are unlikely in both the lower York River (YRKPHa) and Mobjack Bay (MOBPHa), 
but both eutrophication and low dissolved oxygen may affect benthic communities 
in these segments, as well as in the lower York River (YRKMHa). 
Rappahannock River 
All of the degraded samples in the upper Rappahannock River (RPPTFa) were clas¬ 
sified as contaminated. Only five samples were collected in the middle 
Rappahannock River (RPPOH), making assessments of benthic community degra¬ 
dation unreliable. In the lower Rappahannock River (RPPMHa), 67% of the 
degraded samples were classified as contaminated, with an average contaminant 
group posterior probability of 0.67. The remaining degraded samples that were not 
classified into the contaminant group had insufficient abundance/biomass. Only 
eight samples were collected in the Corrotoman River. One of these samples was 
classified as contaminated and another as uncontaminated with insufficient abun¬ 
dance/biomass. 
In summary, degradation in the upper Rappahannock River (RPPTFa) appears to be 
the result of anthropogenic contamination while degradation in the lower Rappa¬ 
hannock River may be the result of a combination of contamination and low 
dissolved oxygen effects. The small number of samples collected makes assessments 
of overall benthic community condition in the middle Rappahannock River 
(RPPOHa) and Corrotoman River (CRRMHa) difficult but, the degradation observed 
appears to be from a variety of sources in both segments. 
Potomac River 
Fifty percent of the degraded samples in the upper Potomac River (POTTF) were 
classified as contaminated by the diagnostic tool. None of the uncontaminated 
degraded samples had excessive or insufficient abundance/biomass. In the middle 
Potomac River (POTOH), 80% of the degraded samples were classified as contami¬ 
nated. Of the uncontaminated degraded samples, 20% had excessive 
abundance/biomass and none had insufficient abundance/biomass. In the lower 
Potomac River (POTMH), 31% of the degraded samples were classified as contam¬ 
inated. Of the remaining degraded samples classified as uncontaminated, 65% had 
insufficient abundance/biomass while only 2.6% had excessive abundance/biomass. 
In summary, benthic community degradation in much of the upper Potomac River 
(POTTF) appears to be the result of anthroprogenic contamination. In the middle 
Potomac River (POTOH), the primary source of stress appears to be contamination; 
appendix k 
2006 303(d) Assessment Methods for Chesapeake Bay Benthos 
