Tier II Stream Segment (MDE)
The Clean Water Act requires three components to water quality standards that set goals for and protect each States' waters. The three components are: (1) designated uses that set goals for each water body (e.g., recreational use), (2) criteria that set the minimum conditions to support the use (e.g., bacterial concentrations below certain concentrations) and (3) an antidegradation policy that maintains high quality waters so they are not allowed to degrade to meet only the minimum standards. The designated uses and criteria set the minimum standards for Tier I. Maryland's antidegradation policy has been promulgated in three regulations within the Code of Maryland Regulations (COMAR): COMAR 26.08.02.04 sets out the policy itself, COMAR 26.08.02.04-1, which is discussed here, provides for implementation of Tier II (high quality waters) of the antidegradation policy, and COMAR 26.08.02.04-2 that describes Tier III (Outstanding National Resource Waters or ONRW), the highest quality waters. No Tier III waters have been designated at this time. Tier II antidegradation implementation has the greatest immediate effect on local government planning functions so the Maryland Department of the Environment (MDE) has prepared this set of Tier II GIS data layers to provide technical assistance to local governments working to complete the Water Resources Element of their comprehensive plans as required by HB 1141. As part of this process, MDE has created this dataset representing the official record of all Maryland Tier II (high quality) stream segments as determined by MDE, the regulatory agency responsible for identification and listing of Maryland's Tier II waters. This dataset consists of a digital geospatial representation of all identified Tier II segments which includes those stream segments promulgated in (COMAR) 26.08.02.04-1, and those additional segments proposed during the current Triennial Review of Maryland Regulations, known as the pending list. Pending segments are Tier II segments awaiting promulgation. This is a vector line file that was developed using the 24,000:1 scale National Hydrography Dataset (NHD) coverage for Maryland, and each identified Tier II stream segment has been linked to the NHD using the unique common identifier (COMID) code. MDE uses Maryland Biological Stream Survey (MBSS) data for designating streams as Tier II. Using all MBSS stations sampled within a stream reach (defined as a section of stream from confluence to confluence), an arithmetic mean of the benthic index of biotic integrity (IBI) and the fish IBI is calculated. Only if the means of both the benthic and fish IBIs are greater than or equal to 4.00 is a stream reach designated as Tier II. As such, Tier II streams represent the best streams in Maryland in terms of water quality, water chemistry, habitat, and biotic assemblages. Tier II stream segments can range in length generally terminating at confluences, impoundment outfalls, and tidal boundaries. However, in planning activities, one should consider the entire upstream watershed to a Tier II stream as any changes to this watershed can potentially have an effect on the water quality of the Tier II stream. It is worth noting that once a stream segment is designated as Tier II, this designation lasts in perpetuity regardless of changes in water quality or local landuse. The publicly maintained list of all Tier II waters and for further information regarding Maryland's High Quality Waters, Tier II, please visit http://www.mde.state.md.us/ResearchCenter/Data/waterQualityStandards/Antidegradation/index.asp
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