River Types: The Spring Hole
Trout have thrived since the glaciers retreated
from North America and Asia some 12,000 years ago. Since then,
these remarkabe, beautiful fish have occupied most available cold
water niches, and evolved into 3 distinct genera,
radiating into more than 23 recognized species,
world-wide. (see: "Trout and Salmon of North America".
Robert J. Behnke. The Free Press, New York) However, their aquatic
environment is governed by conditions that would challenge even
the most well-adapted freshwater animal. Trout have survived the
ravages of floods, extreme reductions in flow rates, ice-outs,
excessively high temperatures, variable food sources, predation,
and competition from other
fish species that feed on similar macro-invertebrates. In fact,
trout are among the few aquatic life forms so hardy that they
live wherever they are introduced, provided that the conditions
are adequate. Only high temperatures, low pH (4.5 and lower),
an absence of sufficient quantities of dissolved oxygen, or toxic
pollutants (e.g., heavy metals such as arsenic, aluminum) discourage
trout from colonizing a given aquatic habitat.
While it is true that some species are hardier
than others, the above statements are valid for nearly all of
them. Based on their resiliency
in any given aquatic habitat, particularly freestone rivers, and
their great adaptive capacities for thriving in new environments,
it should not surprise us that when things get tough, trout usually
prove to be tougher.
Consider fluctuating water temperature as a challenge
to their adaptability. In summer, water temperatures in some freestone
streams may reach up as high as 85°F for short periods, reducing
the concentration of dissolved oxygen well below tolerance limits
for all species of trout. Under these conditions, they do just
what we would do, and seek out cooler environs. This means they
move either to the mouth of a small feeder stream, or into a spring
hole within the same river. Spring holes occur where the interface
between permeable and non-permeable rock occurs at the level of
the riverbed. The water in the porous rock (aquifer)
traverses laterally, and by gravity feeds out into the main current
where the aquifer intersects the river bottom. Spring water is
typically around 45-52°F, and can dissolve more oxygen than
the surrounding warmer water. In contrast, during winter months,
spring holes attract fish because its waters are now warmer than
the main river, which can be as low as 32.2°F.
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Acid Factory Pool |
It is in the summer that groundwater seeping
into the stream saves many fish from suffocation. Almost every
large pool on most freestone rivers has a spring in it, and during
low water times when the temperature routinely exceeds 75°F,
large aggregations of fish can often be observed there. However,
while they avoid one hazard by remaining in the spring hole, they
encounter several others; namely predation and starvation. Blue
herons, mergansers, otters, eagles, ospreys, and the seemingly
ever-present poacher find them easy prey at this highly vulnerable
time in their lives. In order to feed, trout must wait for the
cool of the night air to gradually lower the water temperature
in the main stream. It is only then that they can venture forth
to eat whatever food items remain in the stream. Dawn and rising
water temperatures draw them back again to their refuge. Thus,
if the time it takes the river to cool is not sufficiently long
enough to allow the fish to forage freely, they rapidly loose
weight and soon succumb to the ravages of starvation. During periods
of drought, if high temperatures continue for longer than a week
or two, springs flow slower, and massive trout kills can result.
During this stressful time, larger fish survive by crowding out
the weaker, smaller ones.
Angling in these places also poses a serious
threat to all the fish, regardless of their size. Any fish hooked
in a spring hole rushes into the surrounding warm currents in
its attempt to get free, and is almost certain to eventually die
of either heat exhaustion or starvation by depleting already limited
fat reserve, even If it appears to be normal after being released.
In fact, rivers should be closed during these times, a policy
rarely implemented where such events are a regular occurrence.
Houses and campsites along the river that rely
on wells as the sole source of water can lessen the flow of the
aquifer to the stream. Commercial bottling of water collected
from tapping into the river’s aquifer, or from a spring
high up in its headwaters, places fish below in the main stem
of the river at risk, and such activity could potentially reduce
summer flows below tolerable limits. Protecting trout habitat
includes understanding their biological needs. For the most part,
it’s just plain common sense. Their health, as with ours,
begins with an ample, constant supply of clean, cold water.
