11-2, and Table 11-1 below). The remaining tidal segments in Chesapeake Bay were characterized 
as having the open-water designated use year-round. 
Table II-l Chesa 
peake Bay segments with assigned designated uses. 
Designated 
Use 
Segment Code 
Location 
Deep Water 
and Deep 
Channel 
CB3MH 
CB4MH 
CB5MH 
CHSMH 
EASMH 
PATMH 
POTMH 
RPPMH (portion S of UTM Y = 4185000) 
Chesapeake Bay Mainstem 
Chesapeake Bay Mainstem 
Chesapeake Bay Mainstem 
Chester River 
Eastern Bay 
Patapsco River 
Lower Potomac River 
Lower Rappahannock River 
Deep Water 
Only 
CB6PH (portion north of UTM Y 4145) 
CB7PH (portion N/NW of UTM Y = UTM X + 3752745) 
PAXMH 
SBEMH 
YRKPH 
Chesapeake Bay Mainstem 
Chesapeake Bay Mainstem 
Lower Patuxent River 
South Branch Elizabeth River 
Lower York River 
Source: U.S. EPA 2004 
The EPA Chesapeake Bay Program Office acknowledged this computation code improperly 
imposed pycnocline presence at times and places where none was found. Such applications of a 
long-term mean pycnocline instead of no pycnocline were, therefore, incorrectly applying 
dissolved oxygen criteria assessments in such situations. The EPA published procedures, as 
described below, allow for the presence of episodic pycnoclines. 
Episodic Pycnoclines Criteria Assessment Protocols Modification 
The dissolved oxygen criteria assessment methodology is now clarified to specifically allow the 
deep-water and deep-channel designated uses to occur “episodically” for those 13 segments that 
have been identified as having deep-water and (or) deep-channel designated uses (see Table II-1 
in U.S. EPA 2004) When a pycnocline is observed during the tidal water quality monitoring 
cruise within one of the 13 segments during June 1 through September 30, the deep-water and 
(or) deep-channel designated uses exist and their respective numeric dissolved oxygen criteria 
are applied to those uses. When no pycnocline is observed, the open-water designated use applies 
to the entire water column. By definition, this approach eliminates the default use of long term 
pycnocline average when no pycnocline is observed. 
Reassessment of previous dissolved oxygen assessments by EPA and its state and District 
partners showed only small changes in Chesapeake Bay dissolved oxygen criteria attainment 
results over time. Times and places where no pycnocline could be defined for summer season 
among the 13 Chesapeake Bay segments with previously defined deep-water and deep-channel 
designated uses were shown to be rare events. 
10 
