The Illustrated Field Deployment Guide for Rivers and Streams

2. Site Selection

The sampling site location is dependent on the following factors:

  • Project objectives - what will your data represent? Water-quality characteristics of a stream in a reach of interest? Inflow and outflow from an impounded body of water? There are many reasons to monitor the water quality in a river; being mindful of the sensor location in relation to your objectives is the first consideration in site selection. For the purposes of this guide, it is assumed that you have selected a general location for your sampling purposes. The rest of this section will focus on considerations to make when selecting the actual location of your sensor in the stream.

2.1 Location within channel/reach

Aerial photo of river in sampling vicinity
The aerial photo above show some examples of conditions that must be considered when selecting a sensor location. The key word is "representative". The station location must be representative of the water body, or its particular characteristic of interest. Installation, access, and stability of installation are important and safety is critical; however, if the location of the sensor does not represent the water body you are attempting to monitor, all efforts and expenses cannot provide the needed data.

  • Cross-section variation and vertical stratification -- Cross-section surveys
    • In streams, make cross-section surveys of the desired parameters to determine the most representative location for monitor placement. Make sufficient measurements at the cross section to determine the degree of mixing under different flow conditions and to verify that cross-section variability does not exceed what is needed to meet data-quality objectives.
      Making cross-section measurements to determine variation in cross section in a stream
      Photo: Collecting cross-section data to verify that fixed-sonde location is representative of the stream.
  • Channel stability and uniformity
    • Bends, sandbars and eddies are sources of non-uniform flow that can result in areas of erosion and/or aggradation. These areas are not ideal locations for water monitoring.
    • Confluences are typically areas of high turbulence and non-uniform flow due to dynamic mixing of waterbodies. Due to the active mixing, monitoring in confluences is not recommended. Monitor a fair distance downstream to obtain more valid data. Distance downstream will depend on site conditions; visit the site at the range of flows as mixing may vary.
      confluence of two streams
      Photo shows the confluence of two streams, with the stream on the right being much more turbid. Locate sensors far enough downstream to ensure that complete mixing has occurred.

    • Erosion and sediment transport -- Avoid areas of streambank erosion for a stable, robust long-term monitoring station. Sediment transport can introduce problems related to streambed aggradation and sediment build-up on intakes or sondes.
    • Human influences
      -- Bridges, aprons, and other structures -- Turbulence affects sensor performance and increases maintenance needs. Bridges and other structures are important souces of flow disturbance and turbulence. Also, localized heavy erosion can occur downstream of these structures and aggradation can occur upstream.
      -- Outfalls, discharge points, spill-prone areas -- Do not sample immediately downstream of these areas unless specifically targeting their effects.

2.2 Flow and Stage

  • Range of streamflow (from low flow to flood) - adjust sondes vertically or horizontally for extreme flows?
    Photo showing sonde out of water
    Example where low water levels have left the probes nearly out of the water: if installed correctly, the sonde and its protective tube should be easily repositioned to re-submerge the sonde.
  • Velocity of streamflow -- Sites must be safe and equipment installations must be robust enough to operate within the range of expected conditions while allowing sensors to perform at peak efficiency. In most cases this means finding a place where velocity is sufficient to avoid deposition of sediment on the sonde while avoiding extreme velocities that can damage sensors or carry large debris.
  • Turbulence -- Turbulent streamflow aids mixing, but also creates problems in monitoring field parameters such as DO, turbidity, and water velocity. Turbulence may also increase sensor maintenance needs.

Additional Site selection considerations:

  • Weather - In addition to seasonal effects (ice, monsoon rains, deep snow), how will day-to-day weather events effect your ability to get to a site and service it? Will that nice dusty embankment you walk down to access your sensor turn to greasy mud after a rain? Will the rural road where you've found a bridge with light traffic be impassible for days following a snowfall?
  • Human activity - Most sampling sites are near bridges for practical reasons. This also can make them vulnerable to disruption from human activity, both intentional vandalism and accidental damage such as dropping an anchor and snagging a cable. Areas of potentially high human activity (fishing areas, boat docks, etc) should be avoided to minimize tampering with the sensors.
  • Site access - be sure site is accessible during all weather conditions but also be aware of its proximity to popular recreation sites or other areas where accidental or intentional damage can occur from human activity.
  • Sonde access - this can be one of the most difficult aspects of a deployed sensor installation. Because a sonde can require servicing at any time, regardless of river stage or weather condition, access is needed at all stages and in all weather conditions.
  • Cross-sectional access - periodic comparisons of fixed-point sonde readings to readings made at intervals across the entire width of a river are often desirable to ensure that the sensor reading is representative of the river as a whole. Access to the cross section at low, mid, and high stage is important and must be considered.
  • Range in stage and flow - this relates to the three points immediately above (site access, sonde access, and cross-sectional access). Be sure to know and anticipate changing conditions before expensive installations are made that can't be accessed for significant periods.
  • Event-driven WQ changes
  • Fouling - In the site-selection process, the potential for fouling will be based on prior knowledge of the streamwater characteristics overall, combined with the flow conditions at the point where the sensor will reside in the stream. For example, the sensor should be placed in a location that receives steady flow, rather than a slack water area, in order to discourage algae growth or sedimentation.
  • Telemetry and power needs - If you need real-time data, it is possible you will have power demands greater than can be provided with a battery and solar panel. The cost of getting electricity to a site can be quite high.
  • Distance from servicing personnel office - Although this is a consideration that is out of the control it is important to factor in to the site design. A site that is remote from the personnel that service it will be time-consuming and expensive to maintain. It may be cost-effective to spend more upfront on robust installation.
  • Safety - Consider all aspects: depth and current of river for sensor access and wading; traffic on road and bridge; safe off-street parking for vehicle; boat and barge traffic; etc.
  • Permits
  • Repairs

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