When using the plane wave reflection coefficient (impedance condition) to represent bottom interaction for a normal mode calculation, counting the number of eigenfunction zeroes to identify normal modes is impractical. A method for identifying these modes by knowing the characteristics of the reflection coefficient is presented [R. A. Koch, P. J. Vidmar, and J. B. Lindberg, J. Acoust. Soc. Am. (to be published)]. In the absence of attenuation, the mode number is the number of eigenfunction zeros for the water column plus a contribution given by the number of times that, as a function of horizontal wavenumber, the reflection coefficient circles the origin in the complex plane. [This work was sponsored by the Office of Naval Research.]