Onset does not currently offer a flow data logger; however, water level data loggers, however, are commonly used to determine stream flow over time and "peak discharge." ("Discharge" is another way of saying the flow volume through the stream, typically in CFS or cubic feet per second.)
Note that in most cases it is more practical to log water level and convert to flow than to measure flow directly. That is because pressure-based water level sensors are lower power and more reliable than flow sensors. Many flow sensors also have the problem of having to be deployed in the middle of a stream, which means that they will catch a lot of debris.
To convert water level data to flow rate data you will need to come up with a "discharge rating curve." This curve is developed by measuring water level and the corresponding current velocities for a cross-section of the stream. So, for each water level in feet, you will have a corresponding total flow rate in CFS. This takes several trips to the stream to determine the relationship over a range of discharge rates. To determine the total flow, you will need to measure the stream flow rate at several points across the stream and ideally at multiple depths. The guideline for depth if only one depth is measured is to measure it at the 60%-of-depth point. For additional information, see http://va.water.usgs.gov/online_pubs/OFR95-713rev.html
Once you have the discharge rating curve for a site, the conversions from the level measurements to flow can be done in a Microsoft® Excel or Corel® Quattro Pro® spreadsheet.