Stream Discharge Measurement – Wadable Streams
1 Purpose
This SOP describes the procedure for measuring stream discharge (volumetric flow rate) in wadable streams using the USGS midsection velocity-area method with Price Pygmy or Price Type AA mechanical current meters. Discharge data are fundamental to characterizing watershed hydrology, calculating pollutant loads, and supporting water quality assessments (Rantz et al., 1982; Turnipseed & Sauer, 2010).
2 Scope
- Applies to: Wadable streams and channels where personnel can safely stand in the flow (generally depths < 3–4 ft and velocities < approximately 4 ft/s)
- Does not apply to: Non-wadable rivers requiring bridge, cableway, or boat measurements; acoustic Doppler instruments (see SOP-F202); or flume and weir installations
- Location: All EA Program stream monitoring sites
3 Responsibilities
| Role | Responsibility |
|---|---|
| Lab PI | Approve measurement sites and protocols |
| Field Lead | Select cross-section locations, verify equipment |
| Trained Personnel | Perform measurements and record data per this SOP |
| New Students | Complete training before making independent measurements |
4 Definitions
- AquaCalc: A digital flow computer that counts current meter revolutions and computes velocity automatically; eliminates manual timing with a headset and stopwatch and reduces transcription errors.
- Cross-section: The plane perpendicular to the direction of flow at which discharge is measured; ideally straight, uniform, and free of eddies or dead zones (Rantz et al., 1982).
- Discharge (Q): The volumetric flow rate of a stream, expressed in cubic feet per second (cfs) or liters per second (L/s); computed as the sum of the products of water velocity, depth, and width across all subsections of the cross-section (Rantz et al., 1982).
- Midsection method: The standard USGS computational approach in which the discharge of each subsection is calculated as the product of the measured velocity, the depth at that vertical, and the horizontal distance to the midpoints on either side of the station (Rantz et al., 1982; Turnipseed & Sauer, 2010).
- Price Pygmy meter: A small-cup mechanical current meter (5 cm diameter) used in shallow water (< 1.5 ft depth) or when velocities are low; 1 revolution = 1 click per second at approximately 0.1 ft/s (Rantz et al., 1982).
- Price Type AA meter: The standard USGS mechanical cup-type current meter (12.5 cm diameter) used in deeper flows; more accurate than the Pygmy for velocities above approximately 0.2 ft/s (Rantz et al., 1982).
- Six-tenths-depth method: Placement of the current meter at 0.6 of the total depth below the water surface; yields the mean vertical velocity in most natural channels; the standard method for shallow profiles (depth ≤ 2.5 ft) (Rantz et al., 1982).
- Spin test: A quality control check in which the current meter bucket wheel is spun by hand and the time for it to stop is measured; the wheel should spin freely for a minimum number of seconds (see SOP-F201); indicates bearing condition and mechanical integrity.
- Tag line: A measuring tape stretched taut across the stream perpendicular to the flow; anchored at both banks; used to measure horizontal distances to each measurement station (Rantz et al., 1982).
- Thalweg: The line of deepest flow along the channel axis; usually the zone of highest velocity and the target for closely spaced measurement stations.
- Two-point method: Placement of the current meter at 0.2 and 0.8 of the total depth; the arithmetic mean of the two velocities gives the mean vertical velocity; preferred when depth > 2.5 ft (Rantz et al., 1982).
- Wetted width: The horizontal distance across the stream from water’s edge to water’s edge at the measurement cross-section.
5 Safety and Hazards
- Drowning and swift water: Do not wade in flows deeper than approximately waist height or where you feel unstable. Use a wading rod as a support. Always face upstream while wading.
- Slippery substrates: Cobble, gravel, and algae-covered rocks are extremely slippery. Wear felt-soled or studded wading boots.
- Hypothermia: Prolonged immersion in cold water can cause hypothermia. Wear insulated waders in cold conditions.
- Biological: Waterborne pathogens may be present. Avoid swallowing water. Wash hands and equipment after each site visit.
5.1 Required PPE
5.2 Emergency Procedures
If a team member loses footing, release the wading rod and equipment and focus on self-rescue by floating feet-first downstream. Never attempt a discharge measurement alone; a minimum of two people must be present. Refer to SOP-F001 Field Safety.
6 Equipment and Materials
| Item | Notes |
|---|---|
| Price Pygmy or Type AA current meter | See SOP-F201 |
| Top-setting wading rod | Graduated in tenths of a foot |
| AquaCalc digital flow computer or headset and stopwatch | For counting revolutions |
| Measuring tape (50 m / 150 ft) | Fiberglass; for establishing the tag line |
| Tape stakes or rebar pins | Two; for anchoring the tag line |
| Mallet | For driving stakes |
| Field data sheet (Appendix A) | Printed on waterproof paper |
| Clipboard and pencils | |
| GPS unit or smartphone | For recording cross-section coordinates |
| Thermometer | For recording water temperature |
| Camera | For photo-documenting the cross-section |
7 Estimated Time
| Task | Time Required |
|---|---|
| Site selection and tag line setup | 10–20 minutes |
| Spin test | 5 minutes |
| Discharge measurement (20–30 verticals) | 30–60 minutes |
| Data recording and site breakdown | 10–15 minutes |
8 Procedure
8.1 Site Selection
Select a measurement cross-section with as many of the following characteristics as possible (Rantz et al., 1982):
- Straight channel reach for at least 3–5 channel widths upstream and downstream
- Uniform flow – no eddies, backwater, or dead zones
- Stable streambed (no excessive soft sediment or mobile cobbles)
- Flow perpendicular to the tag line
- Avoid locations immediately downstream of bridges, confluences, debris jams, or sharp bends
- Depth sufficient to submerge the current meter cups; shallow enough to wade safely
Ideal cross-sections are trapezoidal with gradually varying depth. Avoid abrupt depth changes, undercut banks, and boulders that create turbulence (Rantz et al., 1982).
8.2 Establishing the Tag Line
- Stretch the measuring tape across the stream perpendicular to the flow direction.
- Anchor each end with a tape stake driven into the bank.
- Pull the tape taut so it does not sag into the water.
- Record the distance at the left water’s edge (LEW) and the right water’s edge (REW), looking downstream. All station distances are measured from the same reference bank (typically the left bank, looking downstream).
8.3 Determining Station Spacing
Divide the wetted width into subsections. The goal is 20–30 subsections; no single subsection should carry more than 5–10% of total estimated discharge (Rantz et al., 1982).
| Wetted Width | Recommended Station Spacing |
|---|---|
| < 5 ft (1.5 m) | 0.5 ft (0.15 m) |
| 5–15 ft (1.5–4.5 m) | 1.0 ft (0.3 m) |
| > 15 ft (4.5 m) | Divide into 20–30 equal intervals |
Space stations more closely near the thalweg where depth or velocity changes rapidly.
8.4 Assembling and Testing the Meter
- Mount the current meter on the wading rod per SOP-F201, Section 7.1.
- Connect the cable to the AquaCalc or headset.
- Perform a spin test and record the result on the data sheet.
8.5 Making the Measurement
Work from one bank to the other. At each station:
- Position yourself downstream of the wading rod; your body must not disturb the flow at the measurement point.
- Record the station distance from the tag line.
- Measure and record the water depth to the nearest 0.01 ft.
8.5.1 Setting the Meter Depth
| Method | Meter Position | When to Use |
|---|---|---|
| Six-tenths | 0.6 × depth from the surface | Depth ≤ 2.5 ft or Pygmy |
| Two-point | 0.2 and 0.8 × depth (average the two) | Depth > 2.5 ft |
| Surface | 0.2 × depth; apply coefficient 0.85–0.90 | Very shallow or weedy |
(Rantz et al., 1982; Turnipseed & Sauer, 2010)
- Count revolutions for a minimum of 40 seconds (up to 70 s at low or unsteady velocities). Record revolutions and elapsed time (or velocity if using the AquaCalc).
- Move to the next station and repeat.
When counting revolutions manually, look up velocity in the rating table for your meter model (see SOP-F201, Section 9.3).
8.6 Edge Measurements
At the left and right water’s edges, record depth = 0 and velocity = 0. These close the cross-section.
8.7 Post-Measurement Spin Test
Repeat the spin test after completing the measurement. A reduced spin time indicates potential bearing damage; the measurement quality is suspect (Rantz et al., 1982).
9 Discharge Calculation – Midsection Method
Total discharge is the sum of discharge through each subsection (Rantz et al., 1982; Turnipseed & Sauer, 2010):
\[ Q = \sum_{i=1}^{n} V_i \cdot d_i \cdot w_i \]
where for station \(i\):
- \(V_i\) = mean vertical velocity (ft/s or m/s)
- \(d_i\) = water depth (ft or m)
- \(w_i\) = width of the subsection = distance from the midpoint to the adjacent station on each side (ft or m)
Report discharge in cfs or L/s, rounded to three significant figures following USGS conventions (Turnipseed & Sauer, 2010):
| Discharge Range | Rounding |
|---|---|
| < 1 cfs | Nearest 0.01 cfs |
| 1–10 cfs | Nearest 0.1 cfs |
| 10–100 cfs | Nearest 1 cfs |
10 Data Processing
- Enter field data into the lab’s standard discharge spreadsheet template on the same day as the measurement.
- Verify the total discharge calculation matches the field computation.
- Flag any station carrying more than 10% of total discharge in a single subsection – this indicates station spacing was too wide.
- Archive the original field data sheet (scan if paper).
11 Quality Control
- No single subsection should carry more than 5–10% of total discharge (Rantz et al., 1982).
- Pre- and post-measurement spin tests must meet minimum thresholds (see SOP-F201).
- If two measurements are made at the same cross-section, they should agree within 5%; if they differ by more than 5%, make a third measurement and use the mean of the two closest values (Turnipseed & Sauer, 2010).
- Oblique flow: If current is not perpendicular to the tag line, velocity readings underestimate true velocity. Reposition or apply a cosine correction.
- Wading disturbance: Stand downstream of the meter; avoid shuffling feet near the measurement point.
- Unsteady flow: In flashy streams, stage may change during measurement. Record the time at each station.
12 References
14 Training Record
Before making discharge measurements independently, personnel must:
- Read this SOP and SOP-F201 in full
- Demonstrate safe wading technique and proper use of the wading rod
- Successfully set up a tag line and conduct a pre-measurement spin test
- Measure a discharge under supervision (minimum one complete cross-section)
- Demonstrate correct midsection calculation
- Be signed off by the PI or field lead
15 Appendix A: Field Data Sheet
Print on waterproof paper (Rite in the Rain). One sheet per measurement.
Site: _______________ Date: _________ Time: _________
Operator(s): _______________ Meter type: ________ Meter #: ________
Pre-measurement spin time: ______ s Post-measurement spin time: ______ s
Water temperature: ______ °C Weather: ____________________
| Station (ft) | Depth (ft) | Meter depth setting (ft) | Revolutions | Time (s) | Velocity (ft/s) | Width (ft) | Q_sub (cfs) |
|---|---|---|---|---|---|---|---|
Total Q: ______ cfs Notes: ____________________________________