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River
Discharge - the rate of flow of water
The flow of water in any stream or
river is a function of inputs to and outputs from the channel. Inputs into
the Portneuf River come from precipitation,
snowmelt, tributaries, and groundwater. The relative importance of these
inputs varies from one year to the next and during the year. Outputs from
the channel are the result of diversions, evaporation, and movement of
water from the channel into groundwater, all of which also vary seasonally
and among years.
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The figure to the right shows
how average flow varies during the calendar year. Data in this and the
following figures are from the USGS Carson
Street Gauge, located in Pocatello.
Flows are typically highest in late spring because of melting snow at
higher elevations and because this is when precipitation
is usually greatest. Flows decrease rapidly during late spring and early
summer. This is a period when inputs from snowmelt and precipitation
decline, and when withdrawals for irrigation are increasing. Flows
typically increase from July through November as precipitation increases
and as withdrawals for irrigation decline.
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Variation in average daily
flows over the past 90 years are illustrated in the next figure. Data in
this figure represent the average daily flow for each year since 1910; the
horizontal line represents the average of those values. There were extended
periods of low flows from 1924 through 1942, from 1953 to 1964, and again
from 1987 through 1995. The first four years of the current century have
also been well below average. In fact, the average flow for 2000 –
2003 is the fourth lowest consecutive four-year average in the past 90
years.
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2003 Calendar Year
This figure shows average daily flow in the Portneuf, in blue, and discharge
in 2003, in red. Peak flows in 2003 occurred much earlier in the year and
were only about half of average peak flows. This was the third consecutive
year in which precipitation was substantially below average, and some
streams that normally flow into the Portneuf, including Rapid Creek, dried
up completely before they reached the Portneuf.
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2004 Calendar Year
Significant snowfall in December
2003 and precipitation in 2004 that was close to average (the first time in
5 years) resulted in increased discharge in the Portneuf. Peak flow, which
occurred almost two months later than in 2003, was slightly above average
discharge for that period, and the minimum discharge was 22 CFS, compared
to 5.7 CFS in 2003. Higher than average precipitation in October caused a
substantial increase in discharge, bringing water levels at the Carson
Gauge to within about 75% of the long term average for the end of October,
and additional precipitation late in the year pushed discharge above the
long term average.
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2005 Calendar Year
There were multiple peaks in
discharge in 2005, and there were several periods during which discharge
remained above average for days at a time. Precipitation in Pocatello was at or
above average for most of the year, contributing to the high flows in the
Portneuf. As was the case in 2004, minimum flows did not reach the low
values observed in 2003.
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