Ch 13.1 Streams & Rivers pg 280 – 282 Almost half of the water that falls to the Earth’s surface eventually ends up in a stream or river (runoff), where it travels overland to the oceans. Streams and Rivers are an essential part in the water cycle and account for most of the erosion of Earth’s surface. A tributary is a stream that runs into a larger stream. The main river will eventually drain into the ocean. A river and all of its tributaries is called a river system. The drainage basin, or watershed, of a river includes all the land that drains into the river either directly or through its tributaries. The largest drainage basin in the U.S. is the Mississippi River system. The high land that separates one drainage basin from another is called a divide. The major divide in the U.S. is called the Continental Divide. Rain falling east of the divide will flow into the Atlantic Ocean and rain falling west of the divide will flow into the Pacific Ocean. The Continental Divide The beginning of a river is known as the source. The end of the river is known as the mouth. As a river reaches its mouth it slows down and deposits its load in a fan shape called a delta. An alluvial fan is similar to a delta, but forms on land where a river emerges from a mountainous area and flows out onto a more gently sloping plain. Rivers may overflow its banks as a flood. The floodwater may cover part or the entire valley floor where the river runs eroding and depositing sediments. This part of the valley floor is called the floodplain. Features of a Floodplain – created by erosion & deposition A river flowing through a floodplain typically winds back and forth in broad curves called meanders. A river wants to find the shortest, straightest way to the ocean, so it will change paths along the floodplain cutting off wide loops (or meanders) leaving behind a curved body of water called an oxbow lake. Elevated ridges along stream banks are called natural levees. River Development The three stages in the development of a river are described as youthful, mature, and old. Most rivers begin in the highlands or mountains. There water sources such as melting snow and ice feed fast-moving young rivers. As the young rivers feed into larger rivers over flatter land they may take on characteristics of mature rivers. As the river approaches the ocean it slows down and becomes wider & flatter taking on the characteristics of an old river. 1. A youthful river has a steep slope, fast-moving water, V-shaped valleys, and many rapids and waterfalls. Rapids & Waterfalls - caused by steep slopes and differential erosion. 2. A mature river has a shallower slope, is slow moving, and forms meanders. 3. An old river moves very slowly, has a nearly flat slope, and has oxbow lakes.