nixpkgs-unstable visible imperatively but not declaratively

I'm trying to install this Python library python3.6-geopandas-0.3.0. And I successfully installed it imperatively with $ nix-env -iA nixpkgs.python3Packages.geopandas However I need to create an expression so that my Python environment includes it. with import {}; python36.withPackages (ps: with ps; [ geopandas ]) Turns out I get installing ‘python3-3.6.4-env’ error: undefined variable ‘geopandas’ I have both nixos and nixpkgs-unstable configured in my system. Not sure if correctly though. I have channel nixos for root and nixpkgs for my user. $ echo $NIX_PATH nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels I would like to make nixpkgs-unstable the default channel for my user, or at least make its packages visible as an overlay, so I could run the above expression successfully.