Streams are bodies of water that have a current; they are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers are all streams. In streams, water always flows downhill, but the form that downhill movement takes varies with rock type, topography, and many other factors. Stream erosion and deposition are extremely important creators and destroyers of landforms. Rivers are the largest streams. People have used rivers since the beginning of civilization as a source of water, food, transportation, defense, power, recreation, and waste disposal. With its high mountains, valleys and Pacific coastline, the western United States exhibits nearly all of the features common to rivers and streams. The photos below are from the western states of Montana, California and Colorado. A stream originates at its source. A source is likely to be in the high mountains where snows collect in winter and melt in summer, or a source might be a spring. A stream may have more than one source. Two streams come together at a confluence. The smaller of the two streams is a tributary of the larger stream (Figure 1.1). The confluence between the Yellowstone River and one of its tributaries, the Gar- diner River, in Montana. The point at which a stream comes into a large body of water, like an ocean or a lake, is called the mouth. Where the stream meets the ocean or lake is an estuary (Figure 1.2). The mouth of the Klamath River creates an estuary where it flows into the Pacific Ocean in California. The mix of fresh and salt water where a river runs into the ocean creates a diversity of environments where many different types of organisms create unique ecosystems. As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a streams headwaters, often high in the mountains, gradients are steep (Figure 1.3). The stream moves fast and does lots of work eroding the stream bed. Headwaters of the Roaring Fork River in Colorado. As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (Figure 1.4). As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat, level area surrounding the stream channel (Figure 1.5). Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do. The ultimate base level is sea level. A divide is a topographically high area that separates a landscape into different water basins (Figure 1.6). Rain that falls on the north side of a ridge flows into the northern drainage basin and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides. A green floodplain surrounds the Red Rock River as it flows through Montana. (a) The divides of North America. In the Rocky Mountains in Colorado, where does a raindrop falling on the western slope end up? How about on the eastern slope? (b) At Triple Divide Peak in Montana water may flow to the Pacific, the Atlantic, or Hudson Bay depending on where it falls. Can you locate where in the map of North America this peak sits?
Output: when two streams come together, it is at this.