5.1: Introduction

Introduction

On the continents, except in the most arid regions, precipitation exceeds evaporation. Rivers are the major pathways by which this excess water flows to the ocean. Over the continental United States the average annual rainfall is about 75 centimeters. Of this, about 53 centimeters is returned to the atmosphere by evaporation and transpiration. The remaining 22 centimeters feeds streams and rivers, either directly (by landing in the channels or running off across the surface) or indirectly, by passing through the shallow part of the Earth as groundwater first. This 22 centimeters represents an enormous volume of water: 5.2 x 108 cubic meters per day (1.4 x 1011 gallons per day).

Rivers are also both the means and the routes by which the products of weathering on the continents are carried to the oceans. Enormous quantities of regolith are produced on the land surface by weathering, and most of this material is transported by rivers to the sea, either as particles or in solution. The other two principal agents that transport this material to the ocean, glaciers and the wind, are minor in comparison.

Rivers and streams (which term you use is a flexible matter of scale) are channelized flow sofwater on the Earth’s surface. The term overland flow is used for non-channelized flows of water, usually less than a few centimeters deep but very widespread. There is a pronounced dichotomy between non-channelized flow and channelized flow. Have you ever walked up a small stream channel to see what happens to it? Its termination is almost always well defined.

Rivers are enormously diverse, in:

• size: varies by many orders of magnitude
• geometry: highly variable
• substrate: bedrock or sediment
• sediment type: sediment size ranges from mud to gravel
• stage of development: young, with rugged topography and rapid change, to old, with gentle topography and slow change
• climate: ephemeral and flashy to very steady

No two rivers look exactly alike, but we can talk about many things that most if not all rivers have in common, like

• how to analyze the hydrology of rivers
• the dynamics of turbulent open-channel flow • the dynamics of fluvial sediment transport

The classic areas of study of rivers are these:

• fluvial hydrology: the study of water as water in rivers
• fluvial hydraulics: the study of the flow of rivers
• fluvial sedimentation: the study of sediment movement and in rivers
• fluvial sedimentology: the study of sediments in rivers (overlapping strongly with the preceding field) and of fluvial sedimentary deposits
• fluvial geomorphology: the study of fluvial geomorphic processes (things like sediment movement, channel changes, broader river-valley processes, drainage-network development) and the long-term evolution of rivers, river valleys, and drainage systems

Some Basic Characteristics of Rivers

• Rivers have a wide range in size (as measured by either water discharge, sediment discharge, or length).
• Rivers have a wide range of water discharge and an even wider range of sediment discharge, as a function of time.
• Rivers are curvy; they are seldom straight for a long distance.
• Rivers don’t stay in one place: they shift laterally in various ways and at various rates, so there’s at least temporary deposition at many places in the river system.
• Most medium to large rivers can keep pace with crustal subsidence or uplift in some reach of the river by erosion or deposition.
• Rivers have a long history. (How does a river start?)