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Water Quality Standards and Low Flow Considerations Randy Pahl, NDEP.

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Presentation on theme: "Water Quality Standards and Low Flow Considerations Randy Pahl, NDEP."— Presentation transcript:

1 Water Quality Standards and Low Flow Considerations Randy Pahl, NDEP

2 Why Low Flow is Important – WQS are set to protect Beneficial Uses through out the expected range of flows (except during extreme low/high flows – Low flow periods and Low flow locations Highest potential for algal growth and depressed DO in rivers – WQS don’t apply if flows are too low NAC 445A.121(8) – “The specified standards are not considered violated when the natural conditions of the receiving water are outside the established limits, including periods of extreme high or low flows”

3 How is Low Flow Normally Set Not defined in regulations 7Q10 Statistics typically used – Low 7-day flows expected to occur once every 10 years – Drawbacks with highly regulated system such as Truckee Best professional judgment to define alternative approach 1994 Truckee River TMDL – Ranked historic flow years from lowest to highest flows - 1 to 17 (1973-89) Selected 2 nd lowest year out of 17 years (~2 out of 20) – approximating the 7Q10 concept of 1 time out of 10 years. 2011/12 WQS Review – Can’t rely on historic flows – Group is proposing to rely on TROM simulations of 100 years of operations – NDEP recommends using the ~10 th lowest year from the simulations


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