SAO PAULO, Oct 19 (Reuters) - Some grain shipments on rivers in Northern Brazil have been halted due to a drought that has sent Amazon river tributaries to the lowest level in over a century, according to a note sent to clients on Thursday by shipping services provider Serveporto.

According to the note, some barge companies "halted navigation on the Tapajos and Madeira rivers, affecting grain terminals, the primary logistics transportation method in the region."

Some areas of the Amazon have seen the least rain from July to September since 1980, and water levels at the port of Manaus, the region's most populous city, have hit their lowest level since records began in 1902.

"The dry season has hit Amazon ports hard, particularly for barges with lower drafts," Serveporto said. "Many convoys are struggling to continue their operations, leading to reduced load capacities," it added.

Hidrovias do Brasil, a barge company that operates on the Tapajos, said in a statement to Reuters that barges continue to run between Itaituba and Barcarena, where they carry fertilizers and grains.

The company noted it introduced "specific operational flexibility" measures due to drafts being lower than historical averages, including use of maneuvering pushers to navigate barges at the shallowest points.

Brazilian grain exporters were diverting a small number of export cargos to southern port terminals instead of northern ports, grain exporters group Anec said on Wednesday.

Northern routes, which have been disrupted by difficulties navigating shallow Amazonian rivers this spring, have been instrumental to helping the country boost corn and soybeans exports over the past few years.

On Wednesday, the government said the Itacoatiara grain terminal owned by Hermasa, an arm of Brazilian grain company Amaggi, and two large container terminals close to Manaus, were operating with reduced capacity.

The government said it would release 100 million reais ($19.88 million) for emergency dredging services in the area "to prevent impacts on the value of freight and delays in the availability of products that are transported through Northern waterways."

Amaggi's Hermasa, which handles grain barges coming from Porto Velho on the Madeira river, did not immediately comment.

Serveporto also said the low water levels are affecting incoming vessels at Santarem port, in Para state, mainly because of draft restrictions there.

"For discharge vessels, we suggest considering anchorage for draft reduction before berthing due to the uncertainty of exact depths upon arrival."

($1 = 5.0312 reais) (Reporting by Ana Mano and Marcelo Teixeira Editing by Marguerita Choy)