<Table> <Tr> <Th_colspan="2"> Blue Nile </Th> </Tr> <Tr> <Td_colspan="2"> Blue Nile Falls (Tis Issat) at Lake Tana </Td> </Tr> <Tr> <Th> Country </Th> <Td> Ethiopia </Td> </Tr> <Tr> <Th_colspan="2"> Basin features </Th> </Tr> <Tr> <Th> Main source </Th> <Td> Lesser Abay / Lake Tana </Td> </Tr> <Tr> <Th> River mouth </Th> <Td> Confluence with White Nile in Khartoum, Sudan, forming main branch of Nile River </Td> </Tr> <Tr> <Th> Basin size </Th> <Td> 325,000 km (125,000 sq mi) </Td> </Tr> <Tr> <Th_colspan="2"> Physical characteristics </Th> </Tr> <Tr> <Th> Length </Th> <Td> 1,450 km (900 mi) </Td> </Tr> <Tr> <Th> Discharge </Th> <Td> <Ul> <Li> Average rate: 1,548 m / s (54,700 cu ft / s) </Li> </Ul> </Td> </Tr> </Table> <Tr> <Th_colspan="2"> Blue Nile </Th> </Tr>

Who discovered the source of the blue nile