Know Your Watershed: Measuring Streamflow

A reservoir manager needs to know when and how much water to release.  A city manager needs to know how much water is available for new growth. An irrigator needs to know if sufficient water is available to open a ditch. A rafter would like to know if a certain river can be run. A wildlife biologist wants to know if a stream has enough water to support a fish population. All these water users need a way to measure streamflow (also called discharge), the volume of water that passes through a stream cross-section.

Figure 1. A cross-section of a stream showing how streamflow can be measured. The flow or discharge through each subsection is determined by multiplying its area (depth x width) in square feet by its corresponding velocity in feet per second. The total streamflow in cubic feet per second (cfs) is then found by adding up the flows in all the subsections. Source: USGS Fact Sheet (Nielson, J.P. and J.M. Norris, 2007, USGS Fact Sheet 2007-3043)

Figure 2. Taking the measurements needed to determine streamflow as illustrated in the diagram in Figure 1. The rod has a scale for measuring depth and a flow meter attached to the rod measures the velocity of the stream. The yellow rod or tape measures the width. Source: USGS report: Small watershed streamflow measurement, Water Resources, 2013

Streamflow measurements are made at several locations in Ouray County. Four of the locations use USGS (U.S. Geological Survey) stream gauges. Do you know where they are? Do you know where other streamflow measurements are made in Ouray County?

The methods of measuring streamflow vary considerably and depend on the size and speed of the stream, and how often the measurement is needed. The method illustrated in Figure 2 gives a relatively accurate measurement of streamflow, but cannot be used to measure flow in a deep and fast-moving stream, and is impractical for providing continuous measurements at short time intervals (like minutes).

Water speed can be determined by simply floating an object along a stream and measuring the time it takes to travel a known distance, but more accurate methods use propeller-type flow meters or very sophisticated doppler instruments that measure the velocity of fine suspended particles moving with the water. The most common instrument or gauge for measuring streamflow is one like that used by the USGS. The USGS stream gauge provides continuous measurements of streamflow in cubic feet per second (cfs) and stream height or stage.

Figure 3. At left is a diagram of a USGS gauge (like those used on the Uncompahgre River in Ouray County) using a stilling well to measure the stream stage. At right is the curve used to convert stage in feet to discharge in cfs. The curve is developed by taking many manual measurements (like in Figure 1) of discharge at a wide range of stages. Once established for a specific stream cross-section, the only measurement needed to determine discharge is the stage height. Stream stage and discharge are recorded on-site at 15-minute intervals and transmitted to a central database where data are then made available online.

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