They say the longest journey begins with a single step. We begin our study of the five Great Lakes by considering a single lake. Pick your favorite, and keep it in mind.
Our immediate goal is to develop a differential equation which models the amount of pollutant in a single lake. The amount of water and the amount of pollutant will both play important roles in our study. To help in our formulation we define the following parameters:
be the volume of water flowing into the lake, in
cubic miles per year.
be the volume of water flowing out of the lake, in
cubic miles per year.
be the concentration of pollutant in the
incoming water, in units per cubic mile.
be the amount of pollutant in the lake at
time t. The initial condition, which must be known, is the amount
of pollutant in the lake at time zero, namely 
This function,
is the variable in this problem.
With these parameters it is possible to formulate a differential
equation for 
the amount of pollution in the lake at time t.
The basic idea for this equation is the same as for all differential
equations, namely that the change in the pollution level per unit of time,
,
is the amount of pollution flowing into the lake each time period less the
amount of pollution leaving the lake each time period. The challenge
in setting up this differential equation is to determine the amounts
of pollution entering and leaving the lake.
Since we have defined 
to be the concentration of
pollutant in the water entering the lake, the annual inflow of
pollution is given by the product of 
and 
.
The amount of pollution leaving the lake per unit time is also the
product of the pollution concentration and the amount of water leaving
the lake. We assume the amount of water leaving the lake is a known
quantity, a quantity we have called 
What we do not know in
advance is
the concentration of pollutant in the outflow at every time t. We
can, however, write an expression for this concentration. It is
. Since 
is the total pollution in
the lake at time t and V is the volume of water in the lake, the
given quotient is the concentration of the pollutant in the lake.
Since lake water flows out of the lake, the quotient is also the
concentration of pollutant in the outflow.
Once we know the amount of outflow (
) and the concentration
of pollutant in the outflow, the total pollution leaving the lake is
given by the product of these two quantities, namely
The total change in pollution within the lake is then given by the differential equation
Once an initial condition is given for 
this equation is in the
form to be analyzed with Stella.