Objects are bundles of data along with the code necessary to manipulate that data. For example, the inflow to a reservoir contains the actual value of the inflow along with the code that will divide that single datum among all of the reservoirs and virgin flow sources upstream.

Objects may contain other objects. For example, a reservoir is a collection of objects or records called slots. Each slot defines a hydrological variable and includes information on how the model should use the datum contained in the variable. Different top level objects (reservoirs, confluences, diversions and tables) have different numbers and kinds of slots, each of which may be used in a different way.

Within RiverSim, some objects have a fixed collection of slots, reservoirs, confluences and diversions. These cannot directly be changed by the user (although their behavior may be modified through the setting of operating policies, initial conditions and input time series).

Tables, on the other hand, are free form objects whose sole purpose is to hold whatever combination of slots that the user desires.

Tables are typically used to hold slots that serve as intermediate variables in some policy algorithm that the modeler wishes to define.