The commands in Hydro work with a single site and measurement and will produce a graph or a table.  They also work with groups of sites and measurements referred to as a Collection.   This makes it easy to produce reports for sites in a catchment without having to repeatedly run the same command.  You can build any number of collections and these are stored in a Project File.  This is a disk file that Hydro and Manager can use to read all the collections at once.

Collections are built through an editor available in Manager or Hydro.  You give the collection a name, and then use the tree control and buttons to build the required combination.  The screenshot shows an example of a "Matrix Collection".  The collection has four measurements and two sites.  Any Hydro command that runs with this collection will show results for the eight possible combinations of site and measurement.

Collections can also be used to control your view of the data file.  Manager and Hydro will build a tree control based on the collections in a project file and show this instead of the normal site tree.  This lets you see just the data that you are interested in, and lay out the tree control in the way you want it.

A collection can also control other people's view of a data file.  For example you can build a collection that a web server can see, and then offer a web page that lets the customer make what they believe is a free choice, but is in fact a controlled view of the data.