Two formulations of the linear stability problem are used in practice: temporal stability analysis and spatial stability analysis. In the present paper both formulations are used in order to analyze linear stability of slightly curved shallow mixing layers. Chezy formula is used to model bottom friction. From a computational point of view the temporal formulation is simpler since the corresponding eigenvalue problem is linear. Spatial formulation leads to a nonlinear eigenvalue problem. Advantages of the spatial formulation are related to the fact that spatial instability characteristics can be directly compared with experimental data. Results of numerical calculations for a hyperbolic tangent velocity profile are presented.