WWRC 89-08
Limnological Properties of a Rocky Mountain Headwater Reservoir
Abstract
Hyalite Reservoir, Montana, was studied to determine
properties of this small, montane, headwater, deep-release reservoir
relative to reservoirs at lower elevations. While retention
times for waters were as brief as 12 d, the mean residency of 40 d
from mid-March to mid-December was within the range reported
for other reservoirs. No significant through-reservoir gradients for
suspended sediments were observed, contrasting to observations for
most reservoirs. Thermal stratification, evident during the first
part of the summer, was disrupted in August by cool, dense tributary
inflows and strong wind-induced mixing. Dissolved oxygen
concentrations paralleled temperature patterns in the reservoir;
lowest average values for both occurred in waters sampled nearest
the outlet. Total phosphorus averaged greater than twice the total
nitrogen concentrations; greatest average concentrations for both
were found in the near-bottom waters nearest the outlet.
Enrichment of nitrogen concentrations in outflow over inflow
waters is hypothesized to occur through nitrogen fixation by
Aphanizomenon flos-aquae. Despite the relatively high quality of
waters from tributary inflows, an algal bloom, chlorophyll a concentrations,
and primary productivity estimates suggested that the
reservoir was mesotrophic. Circulation of waters within the reservoir
was primarily influenced by wind-induced mixing, thermal
gradients, and currents produced by the deep-water outlet.
(KEY TERMS: montane headwater reservoir; nutrient enrichment;
longitudinal flow patterns; trophic status.)
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