About Raging River discharge:
The Raging River is a modest tributary to the much larger Snoqualmie River in western Washington
State, draining the western Cascades. The Raging drains an area of about 35 square miles (90 square kilometers)
and is fed predominantly by groundwater from rainfall and a little snowmelt at the upper end of the drainage basin.
The Raging River's drainage basin has been impacted by a number of human activities, including logging, roadbuilding,
and construction of artificial levees along the lower stretch of the river.
The size of a river can be measured by discharge, the volume of water passing by a fixed
point along the river in a given amount of time. Discharge is typically expressed in either cubic feet per second
(cfs) or cubic meters per second (cms). For example, at the mouth of the Amazon River, the mean discharge is approximately
200,000 cms or 6 million cfs. Discharge is typically calculated by measuring the width and mean depth of the river
channel, as well as the mean velocity. Multiplying these three variables together gives the discharge.
Discharge varies depending upon the amount of precipitation in the drainage basin. Rainfall
saturating the ground flows into thousands of tiny rivulets in the drainage basin, and then into ever bigger tributaries
that make up the intricate branching stream network. At the end of the summer drought, in September, the discharge
on the Raging River at the lone USGS gaging station (about 2 miles from the mouth of the Raging) can be less than
10 cfs. The record discharge at this same gaging station is over 4000 cfs, obviously related to some serious storms
on top of already saturated ground.
The data show discharge as a function of time at gaging station # 12145500 on the Raging
River in late November 1998, a typically rainy period in western Washington when the ground is mostly saturated.
Only part of the hydrograph is shown. The data begin at peak discharge of about 2500 cfs following an intense rainstorm
and illustrate the decrease in discharge as water is flushed out of the system. The rainstorm that generated this
hydrograph dumped a lot of water in the basin, then passed on quickly. Thus the hydrograph is showing the Raging
River's response to a very discrete rainstorm event.
The data show a very well behaved decrease in discharge as a function of time, which can
be modelled by the student using exponential, power or logarithmic models. All three of these functions fit the
data well, though the logarithmic model yields the best shape. A modified exponential model, "logistic decay",
where the exponent decreases with time, might yield some interesting results.
Modelling these discrete storm events is important for quantifying the response characteristics
for different drainage basins. For example, the Raging River has a steep sided basin with a lot of near-surface
bedrock, which is very impervious. Water falling on the basin is quickly transferred to the Raging River; the river
has a very short response time. The student can be asked: how will the parameters of an exponential model (for
example) differ between a short response basin like the Raging and a long response basin? How will the parameters
of an exponential model differ between a small versus large storm event?
Source: United States Geological Survey
Faculty and students are encouraged to study streams and rivers near their college. The USGS provides near real-time
discharge data for thousands of streams around the US. Go to http://water.usgs.gov/realtime.html