Sec. 1. 38 MRSA §464, sub-§13 is enacted to read:

13.__Measurement of dissolved oxygen in riverine impoundments.__Compliance with dissolved oxygen criteria in existing riverine impoundments must be measured as follows.

A.__Compliance with dissolved oxygen criteria may not be measured within 0.5 meters of the bottom of existing riverine impoundments.

B.__Where mixing is inhibited due to thermal stratification in an existing riverine impoundment, compliance with numeric dissolved oxygen criteria may not be measured within that portion of the impoundment below the point of thermal stratification during the time stratification conditions occur, except where such stratification is caused in whole or in part by the operation of a dam.

For purposes of this paragraph, "thermal stratification" means a change of temperature of at least one degree Celsius per meter of depth, causing water below this point in an impoundment to become isolated and not mix with water above this point in the impoundment.

C.__Where mixing is inhibited due to natural topographic features in an existing riverine impoundment, compliance with numeric dissolved oxygen criteria may not be measured within that portion of the impoundment that is topographically isolated.__Such natural topographic features may include, but not be limited to, natural deep holes or river bottom sills.

Notwithstanding the provisions of this subsection, dissolved oxygen concentrations in existing riverine impoundments must be sufficient to support existing and designated uses of these waters.__For purposes of this subsection, "existing riverine impoundments" means all impoundments of rivers and streams in existence as of January 1, 2001 and not otherwise classified as GPA.

Sec. 2. 38 MRSA §465, sub-§3, ¶B, as enacted by PL 1985, c. 698, §15, is amended to read:

B. The dissolved oxygen content of Class B waters shall must be not less than 7 parts per million or 75% of saturation, whichever is higher, except that for the period from October 1st to May 14th, in order to ensure spawning and egg incubation of indigenous fish species, the 7-day