Read the passage and choose the correct answer to each of the questions. At 2,315 miles (3,726 kilometres), the…

Read the passage and choose the correct answer to each of the questions.

At 2,315 miles (3,726 kilometres), the Missouri River is nearly as long as the Mississippi River, into which it flows. Together the Missouri and the Mississippi form one of the world’s longest river systems.

There are seven major dams and dozens of smaller dams on the Missouri. The dams use water from the river to produce electricity. They also create reservoirs, or artificial lakes, behind them. The reservoirs store water that is needed for cities and for irrigation of crops. Irrigation is important because most of the regions around the river have a fairly dry climate.

In addition to producing electricity, the dams on the Missouri are also used to control the water level of the river. Sometimes water is held back to prevent floods. At other times, water is released to keep the water level high enough for barges to float downstream.

By changing the natural flow of water in the Missouri, dams and reservoirs have altered the river’s ecosystem. They have affected the depth and temperature of the water in different parts of the river. Such changes have made it difficult for some species, or types, of plants, animals, and fish to survive. A few wildlife species in the region are now endangered.

The word “They” in the last paragraph refers to _____.