If I had to do it, I would do this:
GeoGraphics[GeoRange -> {{36.05, 36.17}, {-98.7, -98.52}},
GeoRangePadding -> Scaled[0.1], GeoBackground -> "Satellite"]
then, trace the contour of the dam with the "get coordinates" tool, and then use GeoArea
:
damContour = {{36.14216500804076`, -98.66601000992658`},
{36.15131161335255`, -98.6549477815357`},
{36.1501032100147`, -98.62214348441108`},
{36.15000037203102`, -98.60908533742007`},
{36.13193732747872`, -98.58352140011719`},
{36.10128112030604`, -98.57004960135653`},
{36.08009879008038`, -98.6043835918035`},
{36.1172565230945`, -98.61315564049649`},
{36.12831708030723`, -98.62663775701455`},
{36.13202116684048`, -98.64507269213401`},
{36.13870030579935`, -98.6454368865287`},
{36.13708633726377`, -98.6568745786338`}};
GeoGraphics[{White, Thick, Line[GeoPosition[damContour]]},
GeoRange -> {{36.05, 36.17}, {-98.7, -98.52}},
GeoRangePadding -> Scaled[0.1], GeoBackground -> "Satellite"]
GeoArea[Polygon[GeoPosition[damContour]]]