A rare call has been placed on the main stem of the Yampa River as historic drought conditions continue to strain water supplies across Northwest Colorado, potentially curtailing more than 300 upstream diversions and prompting water managers to urge residents to conserve wherever possible. The call was placed Aug. 19 at the Lily Park Ditch No. 1, Pump Station No. 1, near the confluence of the Little Snake and Yampa rivers. The agricultural water…