The starting point for water accounting is the account window.

If RiverSim has been set up to handle water accounting, it assumes that all water in the system belongs to some account. Each account will have a single owner, who can be an individual, a corporation or a government. Each owner may have any number of accounts.

An account accumulates credits, perhaps shares in system losses, and makes deliveries for some beneficial use. Each of these functions is fully under the control of the modeler and may be varied depending both on the state of other system variables as well as over time.

Each account is divided into one or more bins. Bins are logically separate components of an account, each of which may be in a different place. For example, an account may have water credits in reservoirs on two different branches of a river. The credit and loss policies in each of these two locations would be different, but the account could draw on either of them at will. In this case, the account would have a bin on each of the river branches.

Reservoirs are divided horizontally into zones. In the figure there are two active reservoir storage zones. Zone A is shared by Bin 1 and Bin 2 and Zone B is exclusively owned by Bin 3. The three bins might belong to three different accounts or to a single account. The accounting engine will treat the three bins separately and will credit, debit or assign losses according to the policy that each follows.