Much of the precipitation that falls on land either enters the soil (infiltration) or remains at the surface, moving downslope as runoff. The amount of water that runs off rather than soaking into the ground depends on several factors: (1) the intensity and duration of rainfall, (2) the amount of water already in the soil, (3) the nature of the surface material, (4) the slope of the land, and (5) the extent and type of vegetation. When the surface material is highly impermeable, or when it becomes saturated, runoff is the dominant process. Runoff is also high in urban areas because large areas are covered by impermeable buildings, roads, and parking lots.

Runoff initially flows in broad, thin sheets across hillslopes. This unconfined flow eventually develops threads of current that form tiny channels called rills. Where rills merge, the flowing water creates gullies, which join to form larger stream channels. At first streams are small, but as one intersects another, larger and larger streams form. Eventually they merge into trunk streams that carry water from a broad region to an outlet, such as an ocean or other large body of water.

Drainage Basins

Every stream drains an area of land called a drainage basin or watershed (Figure 1). Each drainage basin is bounded by an imaginary line called a divide, something that is clearly visible as a sharp ridge in some mountainous areas but can be more difficult to determine when the topography is subdued. The outlet, where the stream exits the drainage basin, is at a lower elevation than the rest of the basin.

Drainage divides range in scale from a small ridge separating two gullies on a hillside to a continental divide that splits an entire continent into enormous watersheds. The Mississippi River has the largest watershed in North America, draining over 40 percent of the U.S. land area (Figure 2).

By looking at the drainage basin in Figure 1, you can see that slopes cover most of the area. Water erosion on the hillsides is aided by the impact of raindrops and by sheet flow, moving downslope as sheets or in rills toward a stream channel (Figures 3 and 4). Hillslope erosion is the main source of fine particles (clays and fine sand) carried in stream channels.

If you could observe streams in an area similar to that depicted in Figure 1 over several years, you would see many of them lengthen by headward erosion—that is, by extending the heads of their channels upslope. Headward erosion occurs when the surface flow converging at the head of a channel has enough power to cut the channel deeper (downcut). This lowering of the channel creates steeper headward slopes that erode more quickly. Thus, through headward erosion, a valley extends into previously undissected terrain (Figure 5).

A river system includes not only its network of stream channels but also its entire drainage basin. It can be divided into three zones, based on the process that dominates in each. These are the zones of sediment production (where erosion dominates), sediment transport, and sediment deposition (Figure 6). It is important to recognize that sediment is being eroded, transported, and deposited along the entire length of a stream, regardless of which process is dominant within each zone.

Sediment Production

The zone of sediment production, where most of the sediment is derived, is located in the headwater region of the river system. Much of the sediment carried by streams begins as bedrock that is subsequently broken down by weathering and then transported downslope by mass movements and overland flow. Bank erosion can also contribute significant amounts of sediment. In addition, scouring of the channel bed deepens the channel and adds to the stream’s sediment load.

Sediment Transport

Downstream from the zone of sediment production is the zone of sediment transport, where material acquired by a river system is transported through the channel network along sections referred to as trunk streams. When trunk streams are in balance, the amount of sediment eroded from their banks equals the amount deposited elsewhere in the channel. Although trunk streams rework their channels over time, they are not sources of sediment, nor do they accumulate or store it.

Sediment Deposition

When a river reaches the ocean or another large body of water, its flow slows, the energy to transport its sediment greatly diminishes, and, eventually, the sediment is deposited. Most of the sediment deposition occurs at the mouth of the river to create a delta, forms a variety of coastal features through reconfiguration by wave action, or settles far offshore because of ocean currents. Because coarse sediment tends to be deposited upstream, it is primarily the fine sediment (clay, silt, and fine sand) that eventually reaches the ocean. Taken together, erosion, transportation, and deposition are the processes by which rivers move Earth’s surface materials and sculpt landscapes.

Drainage Patterns

Drainage systems, which are interconnected networks of streams, can exhibit a variety of patterns. The pattern that develops depends primarily on the kind of rock present and/or the structural pattern of joints, faults, and folds. Figure 7 illustrates four drainage patterns.

The most commonly encountered drainage pattern is the dendritic pattern (see Figure 7A). This pattern of irregularly branching tributary streams resembles the branching pattern of a deciduous tree. In fact, the word dendritic means “treelike.” The dendritic pattern forms where the underlying material is relatively uniform. Because the surface material is essentially uniform in its resistance to erosion, it does not control the pattern of streamflow. Rather, the pattern is determined chiefly by the direction of slope of the land.

When streams diverge from a central area like spokes from the hub of a wheel, it is said to have a radial pattern. (refer to Figure 7B). This pattern typically develops on isolated volcanic cones and domal uplifts.

A rectangular pattern exhibits many right-angle bends (refer to Figure 7C). This pattern develops when the bedrock is crisscrossed by a series of joints and/or faults. Because these structures are eroded more easily than unbroken rock, their geometric pattern guides the directions of valleys.

A trellis pattern is a rectangular drainage pattern in which tributary streams are nearly parallel to one another and have the appearance of a garden trellis (refer to Figure 7D). This pattern forms in areas underlain by alternating bands of resistant and less-resistant rock.

• The land area that contributes water to a stream is its drainage basin. Drainage basins are separated by imaginary lines called divides.

• As a generalization, river systems tend to erode at the upstream end, transport sediment through the middle section, and deposit sediment at the downstream end.

• A stream erodes most effectively in a headward direction, thereby lengthening its course.

• dendritic pattern: A stream system that resembles the pattern of a branching tree.

• divide: An imaginary line that separates the drainage of two streams; often found along a ridge.

• drainage basin: The land area that contributes water to a stream. Also called a watershed.

• headward erosion: The extension upslope of the head of a valley or stream due to erosion.

• radial pattern: A system of streams running in all directions away from a central elevated structure, such as a volcano.

• rectangular pattern: A drainage pattern characterized by numerous right-angle bends that develops on jointed or fractured bedrock.

• trellis pattern: A system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.

• watershed: The land area that contributes water to a stream. Also called a drainage basin.