getting started on our tour, there are a few basic facts about Seven Mile Creek
Watershed that may help you better understand the issues.*
Watershed in a Nutshell...
Topography and Land
The upland region is very
flat (0-2% slope) and it comprises most of the watershed's land area.
Land use in this region is dominated by agriculture, and cultivated
fields cover 86% of the region. The second most expansive land cover,
deciduous forest, makes up 6% of the total watershed area and is restricted
predominantly to the riparian zones bordering the creek. This portion
of the watershed is much steeper (40-60% slope). The remaining areas
within the watershed are categorized as wetland, grassland, or farmstead.
Click model of watershed topography
to enlarge image
Seven Mile Creek is a small but
very steep stream. On a 6.1 mile trip from its "headwaters"
in the uplands down to its mouth at the Minnesota River, the creek
drops 210 feet. This gives it a gradient of 34.4 feet/mile. For comparison,
the Mississipi river drops less than 1000 vertical feet over its 2,350
mile journey to the Gulf of Mexico, a gradient less than 0.4 feet/mile.
Gradient is an important factor in understanding flow velocity within
a stream, which controls the stream's ability to erode banks, carry
sediment, and incise a deeper channel.
Click graph to enlarge image
The oldest rock outcropping
in the park is Jordan sandstone, which forms the bluffs at the entrance
to the park. This sandstone formation is overlain by glacial deposits
from the most recent glaciation of this area (14-10,000 years ago).
These glacial tills produced soils, which are characterized predominantly
as poorly drained clay loams. Kasota stone, a decorative dolostone
(part of the Oneota member of the Praire Du Chien Formation) mined
across the river from immediately atop the Jordan, is not present
in the park. It was eroded by rivers that incised the ravine within
which the present-day Seven Mile Creek flows. Though SMC's path was
originally cut by rivers well before the last ice age, the gouging
out of the great Minnesota River valley by water released from glacial
lake Agassiz is responsible for SMC's steep gradient. Over the last
10,000 SMC has been incising down through the Jordan in an attempt
to find the shortest path to the lower-lying Minnesota River.
Click image to view regional occurrence of Jordan
Seven Mile Creek Watershed
is continental, with cold dry winters and warm wet summers. Climatic
records show an average monthly temperature of 46.2 F, and annual
rainfall averages 28.91 inches per year. April to September is considered
the growing season, and most rainfall is observed from April to July.
Click on the image to view average
rainfall data for SMC Watershed