Thermal Enhancement of the Lower Mountain Fork River Trout Fishery

            

The Oklahoma Department of Wildlife Conservation’s (ODWC) Fish Division manages a year-round trout fishery in a 12-mile reach of the lower Mountain Fork River below Broken Bow Dam.  This fishery provides a high quality angling opportunity for trout in the scenic Ouachita Mountains of southeast Oklahoma.  However, seasonal atmospheric heating of the stream coupled with the timing, duration and temperature of hydropower and sluice releases from the dam limit the potential of the trout population and subsequent angler success.
 

The ODWC Fisheries Division, in cooperation with the US Army Corps of Engineers and Southwestern Power Administration, used a river temperature and hydrodynamic model to determine water releases needed to maintain suitable stream temperature (<68 degrees F) in concert with other reservoir and hydropower objectives.  The seasonal availability of cold water in Broken Bow Reservoir (BBR) and the life-span of this resource during a range of reservoir operations were also assessed.  Results from these projects showed that BBR has enough cold water to support the year-round fishery.  However, there is a mechanical inability to release the coldest water from the depths of BBR regardless of the operational strategies used.
 

BBR thermally stratifies during summer causing a difference in elevation between the cold water layer and the intake structures.  This is most notable during dry periods when the elevation of BBR is low.  In addition, the behavior of water as it approaches the intakes negatively influences release temperatures.  These issues all complicate management of the trout stream during warm months because release temperatures are too warm for trout even though inaccessible cold water exists.
 

The temperature management challenges in the lower Mountain Fork River culminated during summer 2005 causing an interruption in operation of the fishery.  During this time BBR was very low and the temperature of sluice releases exceeded that tolerated by trout.  Stocking of the stream was temporarily postponed until cooler weather solved this problem.  Such breaks in the stocking schedule interrupt angling at the stream and have a ripple effect on the local economy which depends on visitation attributed to trout fishing.

 

spillway structure at Broken Bow ReservoirBased on these insights, it is apparent that improving thermal characteristics of this fishery over time and a wide range of circumstances is necessary.  This will involve enhanced access to and subsequent release of colder water from deep in BBR.  Technology for selective withdrawal is available to accomplish this, but the impacts and feasibility of the various options needs to be assessed.  A water quality model of Broken Bow Reservoir is currently being developed that will help managers understand how selective withdrawal will affect water quality in the impoundment and tailwater.  If these results are favorable a feasibility study of tractable improvement options will be conducted.  Finally, implementation of the most feasible alternatives will occur if adequate funding and support from stakeholders and cooperating agencies exist.

 

Spillway structure at Broken Bow Reservoir