Flathead Delta Erosion BACKGROUND Flathead Lake is located in Northwest Montana in what is known as the Flathead Valley. The lake is a major catchment for the water and sediment moving through the Upper Flathead basin. The Upper Flathead Basin extends along what is called the rocky mountain trench, which is a geologic feature consisting of north\south trending normal faults that create a chain of half grabens, steering rivers and entraining water for the lake and the basin. During the last ice age (ending ten to twelve thousand years ago) the trench steered lobes of massive glaciers in to the Rocky Mountains from the north. Gravel deposited in the glaciated valleys of the basin creates today's aquifer. The glaciers reached at least as far as Polson, Montana where a huge moraine was formed. The location of present day Flathead Lake was one of the last pieces of ice to melt when the glaciers retreated, keeping the lake basin from being filled with gravel. The moraine created a natural dam, which in turn created Flathead Lake. Presently, Flathead receives drainage from six major river systems which cover an area of around 18,378 km². Figure 1. is a map of Montana with the Flathead Basin highlighted in light Blue. The Flathead Basin is a smaller portion of the Columbia River basin which drains the Pacific Northwest. GEOLOGICAL In the mid 1980's an EPA funded project set out to study and determine the nutrients and metals in the sediments of Flathead Lake. The idea for the project was to establish baseline scientific data for the lake so impacts from increased population and future mineral, petroleum, and timber harvesting could be monitored. The lake water was sampled and sediment cores were collected for analysis. Along with the ladder, 2D seismic profiles were used to help in sediment correlation analysis. One aspect of the study focused on the delta that is formed by the Flathead River where it enters the lake. Interestingly, the morphological characteristics of the delta did not fit the established characteristics of typical lacustrine (lake) deltas. Specifically, the typical delta for this setting is a complex channel system that develops vegetated deltaic plains, which reach out in to the lake. The deltaic plains typically are vegetated and subaerial except for during maximum spring runoff. The Flathead Delta is quite different. There is a small lobe of vegetated delta, which extends out in to a long sandy plain that is sub aqueous except when the lake level is lowered in the winter (see Figure 2). This is significant because these deltaic plains serve as natural nutrient sinks. The buffering effect of the delta is a key component in a natural hydrologic system. Spring runoff is usually very rich in nutrients that are carried in the water. The delta filters portions of the excess nutrient and deposits it throughout the delta. This process makes deltas very important in the scheme of nutrient cycling. Historic air photos reveal that prior to the instillation of Kerr Dam the Flathead Delta system was quite typical for a lacustrine delta. To show the degradation of the Delta the EPA researchers used vertical air photos from 1937, 1954, 1972, and 1981. The air photos were mosaiced together and photocopied so each mosaic was at the same scale. The delta was then traced by hand from each mosaic. By placing the outlines side by side it is possible to get an idea of how the dam affected the delta (see Figure 3). To further strengthen the presumed impact, lake level elevation hydrographs were drawn representing each year that the air photos were taken. The hydrographs reveal that the lake level regulation inverted the hydrologic cycle of the lake (Figure 4). Evidently the installation of the dam has changed the dynamics of how waves are transmitted as well as the circulation patterns throughout the lake. The loss of the delta plain has had a huge impact not only on sedimentation but wildlife habitat and nesting grounds for many types of birds. Now that almost seventy years have passed since the dam was installed, the lake has been the focus of a lot of study. A new sediment coring campaign was conducted and geo-chemical analysis of the lake continues. To further understand and measure change in the Delta, a GIS has been designed to re-visit the change detection idea addressed in the 1982 study of the lake. Preliminary results of the study are confirming the results of the earlier study and revealing that the dam is continuing to change the face of Flathead.   References: Alt, D., 2001. Glacial Lake Missoula and its Humongous Floods.Mountain Press, Missoula. Deskins, Collins and Moore. Flathead Lake Delta GIS. Ongoing research. University of Montana. 2004 J.Moore, J.Jiwan & C.Murph. Sediment Geochemistry of Flathead Lake, Montana. Department of Geology. University of Montana. Flathead River Basin Environmental Impact Study, U.S. EPA 1982