You can use the traditional method. Throw orange (Standard size) in the river and see how many meters have gone in time.

Without any additional sensor, you can use floats, which you can either observe on a video of your smartphone (you will need a space scale) or for which you define the time of passage in front of a given mark (two people are needed).

You can also record a video of the river with your smartphone and use the ls-piv method for which several references and a free software are available on the internet.

Both methods have uncertainties between 10 and 20%

The USGS typically used current meters. They would measure from a bridge for access and safety, using a cable reel to lower current meter with weight attached. Historically, they would install a cable across the stream and use a manned cable car to lower the current meter with weight. Doppler current meters are very expensive and have trouble in high velocity, turbulent waters. If you had a fluorometer, fluorescent dye could be added at an upstream distance, and monitor and time the fluorescence concentration as it passes downstream in a well mixed section. The oranges may be the least cost, and add them or throw them in at different places in the cross section, as some areas have faster velocity than others. If you have someone throw in 10 oranges upstream 100 meters or a couple meander bends or riffle run pool glide sequences, and use binoculars or can spot each orange at it passes, then you can get an idea of mean velocity. Velocity can vary spatially. Oranges float on or near the surface, which can be the fastest water, but in very turbulent water, some may pass unseen submerged. If the stream passes through a culvert, culverts have flow velocity rating curves that might be helpful. There are some other types of velocity sensors that can be purchased. Without quality equipment and technique in measuring multiple points across the cross section, it would be difficult to expect quality results. If you are trying to get discharge, you want the cross section area for that gauge height times the mean velocity of the cross section, which may average 20 or more readings across the cross section. The depth of the current meter is typically at 0.6 D for shallow waters, and in deeper waters depths of 0.2, 0.8, and perhaps 0.6 D to get an average velocity at that location. The Doppler meter if used, floats on a small boat, and is pulled across the channel to get detailed depth and velocity data. In my circumstance on the upper Chattooga River, it was much cheaper to hire the USGS to help gather the high flow data at our gauging site using their Doppler meter, so we had a quality stage discharge relationship (including velocity if desired). I think we paid $350 per new flood stage reading. The Doppler gauge costs about $25k, and our measurement site was about 2 hour drive from their office, to give you an idea. A rule of thumb—- the stream velocity in feet/sec times the water depth in feet should not exceed the value of 10, for safe wading, wear life jacket, tether rope, backup, dry clothes, etc. as needed. Highly turbulent water, standing waves, etc. best to avoid even considering.

When direct measurement techniques are impractical, current meters are used to gauge the speed of flowing water in ditches, open channels, and streams.

There are a couple of technological solutions that would provide surface velocities. One involves downward looking radar, essentially a Doppler process. The equipment would have to be mounted on a bridge, if one exists. The other, known as Large Scale Particle Velocimetry, uses river bank-mounted video cameras and sophisticated post-processing to produce surface velocities across the stream.

Hello everyone. To give suggestions on speed measurement, it is necessary to describe the original problem in more detail. Do you need to determine the speed only on the surface of the flow or do you need to determine it at different points according to the depth of the flow? A river is a volumetric flow that can be described in Lagrangian or Eulerian representations. From the tasks that you solve, you can use one of these flow views or both views

Thank you all for replying, my original intention was to measure the river flow rather than the flow velocity. Finally, I have tried the dissolution method with good results.

You can measure using Current meter.

There are several ways to measure the flow velocity in a natural high flow river. Here are a few methods:

Current Meters: Current meters are mechanical devices that measure water velocity by using a propeller or impeller that rotates with the water flow. These devices can be anchored to a fixed point on the riverbank or suspended from a cable across the river to measure the velocity at different depths.

Doppler Flow Meters: Doppler flow meters use sound waves to measure the velocity of water flow. They work by emitting sound waves at a specific frequency that bounce off particles in the water and return to the device. The change in frequency of the returning waves is used to calculate the velocity of the water flow.

Surface Velocity Radars: Surface velocity radars use microwave energy to measure the surface water velocity. The radar beam is directed at the water surface, and the return signal is analyzed to calculate the velocity of the water flow.

Tracer Method: The tracer method involves injecting a small amount of a non-toxic tracer substance, such as a dye or salt, into the water and measuring the time it takes for the tracer to travel a known distance downstream. This method can be used to calculate the velocity of the water flow.

It's important to note that measuring flow velocity in a high flow river can be challenging, and the accuracy of the measurements can be affected by various factors such as turbulence, depth, and the type of instrument used. It's best to consult with an expert in hydrology or river flow measurement before attempting to measure flow velocity in a high flow river.

Given the high flow rate of the river, fluorescent dye tracing is a suitable method. This methodology entails the use of a fluorescent dye and a fluorometer. Additionally, if you suspect the discharge rate is less than 3 cumecs, an electroconductivity meter can be used with salt. I hope that this information proves helpful.

Measuring flow velocity in a mountain river at high flows can indeed be challenging and potentially dangerous. However, there are several methods you can use to measure flow velocity safely and accurately. Here are some suggestions

Acoustic Doppler Current Profiler (ADCP): This is a hydroacoustic current meter similar to a sonar, used to measure water velocity. This device can be mounted to a boat or deployed from a bridge or cableway. This method is very accurate and can measure the flow velocity over the entire cross-section of the river, but the equipment is quite expensive.

Remote Sensing Techniques: Techniques like Synthetic Aperture Radar (SAR) and optical remote sensing can estimate surface water velocity. These methods have the advantage of being completely remote, so no one needs to go near the high-flowing river. The resolution and accuracy can be lower than in-situ methods, and they require some technical knowledge to interpret.

Flow Gauging Stations: These are permanent installations that measure the water level (stage) and convert that to flow using a pre-established stage-discharge relationship (rating curve). They require an initial investment in equipment and time to establish the rating curve, but once set up, they can provide continuous, real-time measurements.

Radar or Laser flow meters: These instruments bounce a signal off the moving water surface and measure the returned signal to determine the velocity of the water. They can be mounted above the river and don't require any contact with the water.

Surface floats: In this simple method, you drop a float into the river and time how long it takes to travel a known distance. This only measures the surface velocity, which is typically faster than the average velocity, so you'd need to apply a correction factor. This method may not be feasible or safe in high flows.

ADCP can be used.