So I am a bit stuck on how to start doing this question attached, since there are effectively $4$ lakes then water flow in can be represented by $F_o$ and diversions can be $D$ for variables but I am not sure on how to start build the model for the whole thing
$$\dfrac{dv}{dt} = \text{flow in} - \text{flow out}$$
So for example individually for lake $1$ (Lake Superior) I can write $\frac{dv}{dt} = D_1-F_{o1}$ and for the second lake (lake #$2$) I can write $\frac{dv}{dt} = F_{o1}-D_2-F_{o2}$ and similarly for the other $2$ lakes as well, so how can I create a model for water balance for the whole network?
Any help would be much appreciated!