Flathead
Lake, in northwestern Montana, is the largest freshwater lake west of
the Mississippi River and
is a critical centerpiece of the region from an environmental and
economic
standpoint. Every spring and summer, the volume of water in Flathead
Lake gradually refills from reduced winter levels due largely in part
to the melting
of snowpack in the April to July period. In years where the runoff is
well
below normal, the lake can fail to refill, or maintain, a specified
‘target’
level in early summer. When this occurs, the competing influences of
mandated
downstream flow requirements and lower-than-average inflows create a
situation
where the lake cannot maintain this ‘target’ level. The impact of these
unusually low lake levels places an enormous stress on the local
economy from a
recreational standpoint, which is estimated to average $300,000,000 a
year.
HDR was approached to develop a Drought Mitigation Plan (DMP) for the lake taking into account the various stakeholders in the area. HDR staff was tasked to examine cause-effect relationships of extreme meteorological and hydrological drought in the basin. In the process, HDR examined the relationships between precipitation, various climatic and other physical factors on the peak spring-summer runoff season. HDR was able to utilize the information gathered in the initial phase of the project to produce a seasonal stream flow forecast on relatively short notice. The resulting forecast verified fairly well with and error of less than 5% in a time frame that was over 4 months before the runoff period commenced.
Phase II of the project will encompass utilizing more information in further refining and improving on the accuracy of ‘pro-active’ drought recognition. These results will be combined with parallel HDR efforts in modeling, economic analysis and environmental impact to test options in the DMP. Other efforts will encompass analyzing paleo-climatological data to provide some insight and analysis to the stakeholders on the frequency and severity of drought periods in the past 400 years. The purpose of these efforts will be to attempt to quantify the number of times the DMP would have had to been implemented in the past as an indication of the potential future. Trend analysis techniques will also be employed to given a generalized idea of future hydrological-meteorological conditions on a decadal basis.