= Solution
<Hydrostatic pressure> at the entrance is $p_w=p_{\rm ref}+\rho_wgh$, with a constant reference <pressure>. Therefore $N=p_i-p_{\rm ref}-\rho_wgh$, and constant overburden <pressure> gives
$$
\boxed{N_t=-\rho_wg\,h_t=-\rho_wg(M_L-Q/S_L).}
$$
A filling lake raises <water> <pressure> and lowers <effective pressure in subglacial drainage>[effective pressure], reducing <material creep> closure. A draining lake has the reverse effect. These signs supply the <pressure> feedback in the coupled drainage model.
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