A comparison of modeling and monitoring data to examine model bias is the initial step in the application of the Chesapeake Bay Water Quality Model (CBWQM) to proposed water quality criteria. The proposed water quality criteria include dissolved oxygen (DO), clarity, and chlorophyll, each with fixed concentrations, durations, and frequency within specific regions of the Bay. Data comparison is based on all available mainstem and tributary monitoring data from 1985 to 1994 the full ten years of model calibration. Comparing the CBWQM estimates of DO, clarity, and chlorophyll to observations is used to detect uncertainty in the model calibration. A methodology is described in Section 2, to correct for systematic model errors in the three proposed water quality criteria. The methodology is applied to all regions of the Bay. Each observation of the three proposed water quality criteria is compared to a model computation taken at the same time and location in model space. The comparison is made only for the critical season of each of the proposed water quality criteria, e.g., March-May for the spring criteria of chlorophyll, and July-August for the summer criteria of chlorophyll. Comparisons are made for each proposed criteria at each CB Criteria Segment (CBCS). A CBCS is here defined as an area delineated by the planer boundaries of a CB segment, and temporally and spatially by the definitions of the proposed water quality criteria for DO, chlorophyll, or clarity. Table 1 lists the Chesapeake Bay (CB) segments assessed by this report and the associated monitoring stations.