The upper parts of the system have rapids and flooding, and pass through areas where very few people live. The middle section is controlled by a series of large hydroelectric dams. The dams were built to supply a major Russian primary industry. Partly built by gulag labor in Soviet times, industrial contamination is a big problem in the area. The area is very hard for the police to watch. Moving on through sparsely populated taiga, the Yenisei grows with many tributaries and finally reaches the Kara Sea in desolate tundra where it is covered with ice for more than half the year. As with other Siberian rivers, the flow has increased lately. People think this is because of global warming. A concern was also raised that a change in the salt in the Arctic may have a global impact on ocean currents.