declare function local:getRivers( $riverNode as node() ) as node() {
    return (if ($riverNode/source/from[@water]) then (
        let $a := (
            let $ans := (
                for $river in tokenize($river/source/from/@water, '\s')
                let $riverLength := //river[@id = $river]/length
                return <river id="{$river}" length="{$riverLength}"/>
            )
            return <rivers>{$ans}</rivers>
        )
    ) else
        <rivers></rivers>
    )
};

let $Ama := //river[name = "Thames"]
return local:getRivers($Ama)