Stream Base Flow Standard discharge measurement base flow code. A code to describe whether stream flow is base flow or not. United States Geological Survey, Surface Water User Group UNSP 1 true unspecified Base flow is not specified for the measurement. BASE 2 true base flow Streamflow was measured at base flow. NBAS 3 true non-base flow Streamflow was measured above base flow. UNKN 4 true unknown It is not known if the measured flow is at base flow.