To get an idea we only need to look at what elephants consume – and they are much smaller than mammoths. At www.macroevolution.net/what-do-elephants-eat.html … we find they eat as much as 660lbs of vegetation in a day – and they drank as much as 50 gallons of water in a single 24 hour stint. The interesting thing is that because they need to drink so much water, in the wild, they are never far from rivers and water holes. They are even capable of excavating holes in the ground in search of underground water sources – by using their tusks as digging tools. Mammoths had really prominent tusks. Does this support the oft repeated claim that mammoths were adapted to a tundra like environment?
Elephants are both grazers and browsers and they eat plants of almost any size, from grass to the limbs of trees. The elephant trunk is designed to bring food to his mouth whether is is savannah like grass and herbs or eating leaves and twigs from above them. They will eat leaves of trees and shrubs, flowers, fruits, twigs, bamboo etc but their main food is grass (and the thick tufts of grass that grow on the African equivalent of the steppe flora). Therefore, mammoths most likely lived in a savannah environment (the steppes of southern Russia must have reached northern Siberia in the Pleistocene). It may even be that Arctic Siberia, Beringia and parts of Alaska were steppe like rather than tundra – which would account for so many bones piled up in what is now the frozen Arctic shores (and offshore). How could herds of mammoths be sustained on sedges and poor quality grass (which grows slowly in cold climate regimes).