diff --git a/src/crater/module_crater.f90 b/src/crater/module_crater.f90
index 24a64867..79aae302 100644
--- a/src/crater/module_crater.f90
+++ b/src/crater/module_crater.f90
@@ -253,4 +253,14 @@ subroutine crater_make_list(domain,prod,ntotcrat,production_list)
       end subroutine crater_make_list
    end interface
 
+   interface
+      function crater_critical_slope(user,crater,iradsq) result(critical)
+      use module_globals
+      type(usertype),intent(in) :: user
+      type(cratertype),intent(in) :: crater
+      integer(I4B),intent(in) :: iradsq
+      real(DP) :: critical
+      end function crater_critical_slope
+   end interface
+
 end module