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Pleistocene, about 10,000 years ago, the
Niobrara River began to form, capturing
headwater streams in the Elkhorn and Loup
basins. About 8,000 years ago, during arid
periods, the Sand Hills and its streams began
forming with the youngest river in the state
being the Dismal River which may be less
than 1,500 years old.
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In attempting to see how the glaciations
would have affected crayfishes, we are
comparing the current ranges with
information on the maximum extent of the
glaciations. We do not know and will never
know what the ranges would have been like
before the Pleistocene. All we can do is
look at the current information and make our
best guess as to what happened.
Northern crayfish, Orconectes virilis
This map shows that the Northern
crayfish, Orconcectes virilis, is truly a
northern species, with a range extending
from the Continental Divide to the
Atlantic Ocean and from the south-central
U.S. well into central Canada. The blue
line denoting the maximum extent of the
ice shows that most of this range would
have been under the ice during the
Pleistocene. It also shows what other
authors suggested; that the refugia for this
species was the western portion of the
Missouri River basin.
The Nebraska collections of the Northern
crayfish are plotted on the map shown
below. In the Northern crayfish species
account I mentioned that it is likely that
those in the White River basin and the upper
Niobrara River are likely to be recent
introductions. So what does this map say
about their presence in the rest of the state
and possible glacial refugia?
