Answer to Question 1
Answer: Drainage density is increased by greater amounts of runoff, which may result from impermeable soils and/or higher rainfall.
Answer to Question 2
Answer: The Pleistocene glaciers had a profound impact on the United States since the glaciation was relatively recent and the processes that alter these deposits are fairly slow. In areas where there was heavy erosion, soils are thin and bedrock structures are readily visible. In areas of deposition, moraines are still visible along with notable sand and gravel deposits.