The Distant Past: The formation of Grassy Point
When the Duluth Superior Harbor was originally charted in 1861, William Hearding, the
cartographer identified a large thumb of wetland protruding out into the open waters of the
estuary as Grassy Point. Then the area was very likely covered with grasses, sedges cattail and
many other aquatic plants.
What Hearding saw, and what was already known as Grassy Point, was the sunken remains of a
sandy, bay-mouth bar that may have once formed the protective sand spit at the head of the St.
Louis River Estuary. The current bay-mouth bar is formed by Minnesota and Wisconsin Points.
Grassy Point is now some 5 miles from the sandy shores of Lake Superior.
Lake Superior has changed dramatically since the retreat of the glaciers 10,000 years ago. Lake
levels have fluctuated hundreds of feet in elevation. Smaller changes have occurred and continue
to occur since. The St. Louis River estuary is a drowned river mouth. Differing rates of rebound
of the earth's crust are occurring on the Lake Superior basin. This is due to the release of
pressure after the retreat of the glaciers. These glacies were thousands of feet thick and
extremely heavy. The north and east parts of the basin are rising faster than the south and west
parts. For Grassy Point, this means that the water level is increasing at a rate of about one foot
per century, slowly flooding the wetland and the rest of the St. Louis River estuary.
On even shorter time scales, fluctuating water levels due to differences in annual rainfall and runoff
amounts influence the level of Lake Superior. Even wind plays an important role in Lake
Superior's coastal wetlands. The term 'seiche' is used to describe the phenomenon when wind
pushes water toward one shoreline in a large lake, causing an increase in water depth along that
shore and a decrease elsewhere. The St. Louis River estuary is defined by the area where the
seiche results in mixing of the lake and river waters. Here the seiche can push lake water more
than 12 miles upstream.
Annual and daily changes in water levels are important to coastal wetlands around Lake Superior
because they strongly influence the types of plants that can survive in the wetland. Most tree
species, for example, cannot survive prolonged periods of flooding. Therefore, trees are absent
from many coastal wetlands. Similarly, there are aquatic plants that cannot tolerate water that
exceeds a certain depth. These plants' distributions may be dictated more by the size and
frequency of changes in water level than the average water depth.
All these natural processes (and many others such as erosion and sedimentation) have influenced
the formation of Grassy Point over the past 10,000 years. They have similarly influenced the
other ecological resources of the area.