The river was 'discovered' during the 1800's but existed for an unknown period before then.
1800's
in the 1800's
1800
around 1800
It was discovered in the mid 1800's to 1900's.
about 1800 years ago
The infrared ray was discovered in 1800 by Sir William Herschel
1400 - 1800 ad)
1800 years ago
1800's
biomass energy was first discovered in the mid 1800's