Water Quality Standards and Low Flow Considerations

Randy Pahl, NDEP

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”

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 2nd 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 ~10th lowest year from the simulations