Sites that are monitored regularly, such as the Ambient Stream monitoring sites, can sometimes become unavailable for a variety of reasons, such as road/bridge work, flooding, park closures, and so forth.

In these instances, a surrogate or temporary site is sometimes set up. When choosing the surrogate site location, care is taken to make sure that monitoring results collected at the temporary site reflect those that would be collected at the original site. All attempts are made to choose a site that is close in proximity to the original site, and that there are no local factors at the surrogate location that may impact the results, such as nearby outfalls or tributaries.

However, it is also important to make sure users of the data are aware that these are two different sites and any result anomalies should take this under consideration.