Automated Salt Dilution Stream Gauging

Stream discharge is a fundamental measurement in hydrology, yet there is still a high degree of uncertainty, especially when estimating high flows in remote locations.  Stream discharge is rarely continuously measured, but rather a rating curve is developed based on an empirical relationship between stream water height (stage) and the volume of water moving per unit time (discharge) across a range of stages, from low baseflow to high storm flows. Rating curve relationships can be difficult to establish for high flow conditions, as site access becomes challenging during turbulent high flow events, and the timing and duration of a high flow event may not allow for manual discharge measurements. This is especially true for storms that are intense and over a short duration, such as occur on Calvert Island, where the Kwakshua Watershed Program is based.  We developed and installed an automated system to measure stream discharge automatically and have successfully installed seven stations. Check out this great mini-documentary from Hakai called “Go With the Flow

How it Works

Stage is measured remotely via submerged pressure transducers, and stream discharge is determined with the salt dilution method which measures the relative dilution of an electrically conductive solution (salt) from the site of injection to downstream. A known volume and concentration of salt solution is added to a turbulent part of the river, the salt is well mixed in the flow, and electrical conductivity is measured downstream. The more dilute the salt solution is downstream (measured by reduced conductivity), the greater the discharge. A detailed description of salt dilution for measuring stream flow can be found here [pdf].

These auto-salt systems were installed and are maintained in collaboration with the Hakai Institute and Hakai Energy Solutions. Each system employs a series of pumps, pressure transducers, automated delivery system and electrical conductivity sensors. A salt solution is delivered to the stream and its relative dilution is measure with conductivity sensors downstream of the deposit site where a characteristic “salt wave” is recorded. The timing, duration, and peak value of the salt wave provide information on streamflow.  All of this is done remotely, allowing for an unprecedented collection of discharge measurements, allowing us to build entire rating curves over single storms and rapidly updated curves when they shift due to channel changes.

Some readers may be concerned about the ecological implications of adding salt to a freshwater stream. Sodium chloride (common table salt) is non-toxic to freshwater organisms at the concentrations and exposure times typically associated with salt-dilution measurements.  We add 500g of salt to 1000L of stream water when measuring steam flow, or the equivalent of 1 sugar cube to 32 cups of coffee.