Maryland's High Quality Waters (Tier II)

What are Tier II waters?

Tier II, high quality, waters are those that have an existing water quality that is significantly better than the minimum requirements, as specified in water quality standards

Why must Maryland identify Tier II waters ?

Federal antidegradation regulations (40CFR131.12) require states to develop and adopt a statewide antidegradation policy that protects all Waters of the U.S. from degradation. These regulations also require states to maintain the condition of high quality (i.e. Tier II) waters that have water quality that is better than the minimum standard necessary to meet designated uses

The Maryland antidegradation implementation procedures are found in the Code of Maryland Regulations (COMAR) This regulation explains how Maryland identifies Tier II waters, when a Tier II antidegradation review is required for certain State permits and approvals, and how to determine current Tier II water quality status based on new data. The regulation also describes the social and economic justification procedure that would be necessary in order to permit the lowering of water quality in a Tier II water.

How are Tier II streams designated? 

Person in stream with dip net Maryland’s 253 Tier II streams are designated based on biological community scores for benthic macroinvertebrates (or ‘aquatic bugs’), and fish. MDE bases its decision on data collection and analysis procedures that strictly follow the Maryland Biological Stream Survey (MBSS) protocols developed by the Maryland Department of Natural Resources (DNR). The analysis protocol generates index of biotic integrity (IBI) scores for benthic and fish data. Any stream where both independent IBI scores are 4.00 or greater are designated Tier II. Explore the interactive map of designated Tier II waters.

What is assimilative capacity?

Assimilative capacity (AC) is defined as the difference between the Tier II baseline index of biotic integrity (IBI) score and the Tier I water quality IBI threshold. The assimilative capacity analysis allows MDE to evaluate recent MBSS data to determine whether Tier II water quality has been degraded. The Tier I water quality IBI threshold for biological water quality is 3.00. The baseline is the IBI score used to designate a stream as Tier II. On a scale of 1.00 to 5.00, any Tier II baseline IBI score (separately for both fish and benthos) must be greater than or equal to, 4.00. 

Maryland’s antidegradation regulations specify that the water quality of a Tier II water is considered diminished if the AC is reduced by more than 25% from the original Tier II designation baseline. This analysis identifies the Tier II stream’s AC threshold, which is the lowest acceptable benthic and fish IBI scores, after considering natural variability. MDE uses more recent MBSS data collected from the Tier II stream segment to determine if the water quality has been maintained. The AC analysis is a way to measure how much Tier II stream water quality can lower before it’s considered degraded. When data is above the AC threshold, the MDE determines that there is some capacity remaining. Conversely, if there is a decline in scores to a level that is at or below the AC threshold, the stream is determined to have no remaining assimilative capacity.

Currently, 62, or almost 25% of all Tier II streams, have no assimilative capacity. Worth noting is that even if a Tier II water is degraded below the AC threshold, the water is still designated as Tier II. Having no capacity means that there are additional steps required in the Tier II review.

What is the Tier II Antidegradation Review?

The purpose of the Tier II antidegradation review is to prevent degradation to high quality waters as a result of permitted activities. The review applies to Water and Sewer Plan amendment approvals, Nontidal Wetlands and Waterways permits and authorizations, and new or modified National Pollutant Discharge Elimination System (NPDES) discharge permits.

The review process identifies impacts associated with a given regulated activity, and then identifies if there are appropriate alternatives that may avoid these impacts to Tier II waters. If impacts cannot be avoided, then the review identifies reasonable alternatives that may minimize or mitigate impacts within the Tier II watershed. More broadly, the review identifies practices that could be considered along with existing conservation, restoration, and planning activities.

Other Tier II Resources:

Contact Information

Please direct questions or comments concerning Maryland's Antidegradation Program to Angel Valdez at

This page was last updated 09/2021​​​