Measuring the Tullis River in February: Winter Stream Gaging

The gage house on the Tullis River is a steel box the size of a phone booth, bolted to a concrete pier below the county road bridge. Inside, a sensor records the height of the water every fifteen minutes and sends the reading to an office ninety miles away. That height is called the stage. It is the number most people see when they look up a river online, and by itself it does not answer the question anyone actually wants answered, which is how much water is going by.

Delia Krohn is a hydrologic technician, and turning stage into discharge is her job. Discharge is the volume of water passing a point in a given time. The relationship between stage and discharge is written as a rating curve, a graph that converts one to the other. A rating curve is not permanent. A flood can scour the streambed or drop a gravel bar in the channel, and afterward the same stage carries a different amount of water. The only way to know is to go measure the river directly and compare the result to what the curve predicted. Krohn and her partner do that about eight times a year on this reach, and the February trip is the one nobody volunteers for. The measurement rests on a single rule, which is that discharge equals area multiplied by velocity, and the rest of the procedure exists to get reliable values for those two quantities.

In open water, the bookkeeping goes like this. The crew stretches a tagline across the channel and records the width, which at the Tullis bridge is about sixty-two feet. They divide that width into roughly twenty-five sections, none of which should carry more than a small share of the total flow. At each dividing line, called a vertical, they lower a weighted current meter, record the depth, and measure velocity at six-tenths of the depth below the surface. In deeper water they take two readings, at two-tenths and eight-tenths, and average them. Each section then has an area and an average velocity, and multiplying the two gives that section's discharge. Adding the sections together gives the discharge of the river.

Ice changes almost every step. In February the Tullis carries a lid of ice eight to fourteen inches thick, so there is no tagline stretched over open water and no wading. The crew walks the ice with a powered auger and cuts a hole at every vertical. Depth is measured from the bottom of the ice rather than from the surface, because the ice occupies part of the channel and no water flows through it. Velocity readings run lower than they would in summer, since the underside of the ice drags on the water the way the streambed does from below. Slush that collects under the ice, which the crew calls frazil, can block part of the channel and move overnight.

Those conditions make winter numbers less certain than summer ones, and the record says so. Winter discharge at many stations is published as an estimate, and the agency marks it that way. The measurement Krohn made in February came to 214 cubic feet per second, and to give a sense of scale she notes that one cubic foot per second equals 448.8 gallons per minute, a standard conversion used across the field. The rating curve had predicted a somewhat higher figure, so the curve was shifted for the season.

The reason anyone pays for a person to stand on a frozen river with an auger is that other decisions depend on the number. A reservoir operator downstream chooses release rates from it. A flood forecast uses the record to judge how the basin responds to rain. Water rights accounting between irrigation districts is settled with it. A fish biologist arguing that a stretch of river runs too low in late summer needs a defensible measurement rather than an impression.

The path into this work is not a single one. Some technicians start with a high school diploma and learn the equipment through a training program and years of field seasons. Many hold an associate degree in a technical field, and a large share come in with four-year degrees in geology, geography, or environmental science. Hydrologists who design the monitoring networks and write the models usually hold graduate degrees. Krohn took the two-year route, then finished a bachelor's degree at night over six years, and says the field seasons taught her more about rivers than any course did. The work rewards patience with cold hands, careful arithmetic, and the habit of writing down what happened even when it makes the day look bad.

1. According to the passage, what is the difference between stage and discharge?
2. Why does a rating curve need to be checked by direct measurement rather than used indefinitely?
3. Explain how the crew arrives at a single discharge figure for the whole river from measurements taken at separate verticals.
4. The February trip is made on ice. Describe two ways the ice changes the measurement procedure, and explain why each change is necessary.
5. The passage says winter discharge is published as an estimate. Using details from the passage, explain why the winter number carries more uncertainty than a summer one.
6. Write a paragraph of six to eight sentences explaining to a reader who has never seen a gage how a river's height becomes a published discharge figure. Use at least three details from the passage.