Town of Manhattan Water Right Consent EA

The modifications to the permit conditions would temporarily adversely affect instream flow water rights held by FWP for the Gallatin River. FWP’s instream flow water rights protect the fishery and wildlife resources of the Gallatin River. The temporary adverse effect may occur for a period of five years until December 31, 2025, by which time Manhattan plans to complete a project to fully mitigate the impact of the water right permit both in terms of timing and volume. The Department of Natural Resources and Conservation (DNRC) granted Manhattan a water right permit for 575 gallons per minute up to 256.11 acre-feet of groundwater to serve a new subdivision of the Town (Permit No. 41H 30021840). Manhattan intends to use a portion of this permit to serve the Pioneer Crossing and Centennial Village Subdivisions, up to a maximum consumptive rate of 138.21 acre-feet. The use of the new water permit would result in an estimated year-round depletion of the Gallatin River of 0.19 cubic feet per second (cfs) equivalent to 138.21 acre-feet. DNRC included amongst conditions on the water permit a requirement that the depletion of the Gallatin River be mitigated through the infiltration of West Gallatin River water previously used for irrigation into the local aquifer. This water would have replaced the 0.19 cfs depletion caused by the water permit. Manhattan is unable to complete the change of the irrigation right to mitigation using infiltration as required by the permit conditions. They do not have the authority or agreement of the ditch company to use the ditch through which the water historically used for irrigation was transported. Manhattan is unable to transport the water needed to infiltrate to provide for mitigation as required by the permit conditions. This situation resulted in Manhattan petitioning DNRC to modify the permit conditions. The requested modification would remove the permit condition requiring the infiltration of West Gallatin irrigation water to provide for year-round mitigation and would instead leave this water in the West Gallatin River. Additionally, Manhattan would develop a rapid infiltration basin (RIB) to infiltrate treated wastewater from its municipal wastewater system to provide for mitigation of the new water permit during the non-irrigation season. However, Manhattan anticipates it may take through 2025 to complete the RIB. In order to allow for the prompt use of the water permit, which is needed to serve the continued development of the subdivision, Manhattan has requested that until such time that the RIB is completed, that changing the West Gallatin irrigation right to leave the water in the river would be the only means of mitigating the depletion resulting from the use of the water permit. This would result in a temporary adverse effect to FWP’s instream water rights in the Gallatin River during the non-irrigation season when the 0.19 cfs depletion associated with the new permit is not mitigated. This estimated depletion represents the maximum theoretical depletion but is unlikely to occur in full as the subdivision will not likely reach full build-out by the time the RIB is operational. At the same time, it would result in more water in the Gallatin River during the irrigation season than would otherwise be present as the full annual volume needed to be mitigated would be left in the river during the 100-day irrigation season. Because of the non-irrigation season adverse effect to FWP’s instream flow water rights, Manhattan has requested that FWP consent to the modifications of the water permit without consideration to the adverse effect to FWP’s instream water rights until such time the RIB is completed and no later than December 31, 2025. During this time period, FWP would refrain from making call on junior water users during the non-irrigation season to the extent of the 0.19 cfs projected depletion caused by the water permit. This Environmental Assessment (EA) evaluates this request and the no-action alternative. The no-action alternative could result in the developer relying on small wells exempt from water rights permitting. The preferred alternative is to grant Manhattan’s request.

Related Attachments

• Town of Manhattan Water Right Consent

Contact Information

Andy Brummond Water Conservationist Montana Fish, Wildlife and Parks 333 Airport Rd. Ste. 1 Lewistown MT 59457 Or email comments to: abrummond@mt.gov