Topographic maps are used to show natural and physical features of a landscape. Topographic maps include contour lines that designate regions with common altitudes (elevations).
These contour lines would change to represent lower elevations a million years in the future. This would be due to the processes of weathering and erosion.
Weathering is the wearing away of land due to the exposure of environmental conditions. Weathering is classified as being either physical or chemical. Physical weathering includes weathering due to the pounding of the elements (rain, hail, wind), ice wedging, burrowing of animals, etc. Chemical weathering is the breaking down of terrain due to chemical processes such as being deteriorated from acid rain or acids that are released from mosses or lichens. Chemical weathering can also be due to decomposition or oxidation.
Erosion is when the fragments and particles created by the weathering of Earth's terrain are carried to a new location.
Thus, over time, Mt. St. Helen's elevation would be weathered down and its matter would be carried to a new location via erosion. This would result in the mountain being a lower elevation.