= Solution
With constant lake surface <area>, raising its level by $dh$ changes its <volume> by $S_L\,dh$. The symbol $M_L$ in the proposed balance is an effective filling rate measured as lake-level rise per unit <time>, so the <volume> inflow is $S_LM_L$. <Volume> conservation gives
$$
S_L\frac{dh}{dt}=S_LM_L-Q,\qquad\boxed{h_t=M_L-Q/S_L.}
$$
Thus the lake rises when its inflow exceeds channel discharge and falls in the opposite case. If an inflow were instead specified directly as a <volume> rate $I_L$, its contribution would be $I_L/S_L$; keeping these units distinct is necessary for the interpretation of $M_L$.
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