From headwaters to mouth, the physical variables within a river system present a continuous gradient of physical conditions. This gradient should elicit a series of responses within the constituent...

The River Continuum Concept

H umans have long been fascinated by the dynamism of free-flowing waters. Yet we have expended great effort to tame rivers for transportation, water supply, flood control, agriculture, and power generation. It is now recognized that harnessing of streams and rivers comes at great cost: Many rivers no longer support socially valued native species or sustain healthy ecosystems that provide important goods and services (Naiman et al. 1995, NRC 1992).

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The Natural Flow Regime

Publisher Summary This chapter reviews the rates at which Coarse Woody Debris (CWD) is added and removed from ecosystems, the biomass found in streams and forests, and many functions that CWD serves. CWD is an important component of temperate stream and forest ecosystems and is added to the ecosystem by numerous mechanisms, including wind, fire, insect attack, pathogens, competition, and geomorphic processes. Many factors control the rate at which CWD decomposes, including temperature, moisture, the internal gas composition of CWD, substrate quality, the size of the CWD, and the types of organisms involved. The mass of CWD in an ecosystem ideally represents the balance between addition and loss. In reality, slow decomposition rates and erratic variations in input of CWD cause the CWD mass to deviate markedly from steady-state projections. Many differences correspond to forest type, with deciduous-dominated systems having generally lower biomass than conifer-dominated systems. Stream size also influences CWD mass in lotic ecosystems, while successional stage dramatically influences CWD mass in boat aquatic and terrestrial settings. This chapter reviews many of these functions and concludes that CWD is an important functional component of stream and forest ecosystems. Better scientific understanding of these functions and the natural factors influencing CWD dynamics should lead to more enlightened management practices.

Ecology of Coarse Woody Debris in Temperate Ecosystems

Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9 Pg C y−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9 Pg y−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.

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Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget

A huge road network with vehicles ramifies across the land, representing a surprising frontier of ecology. Species-rich roadsides are conduits for few species. Roadkills are a premier mortality source, yet except for local spots, rates rarely limit population size. Road avoidance, especially due to traffic noise, has a greater ecological impact. The still-more-important barrier effect subdivides populations, with demographic and probably genetic consequences. Road networks crossing landscapes cause local hydrologic and erosion effects, whereas stream networks and distant valleys receive major peak-flow and sediment impacts. Chemical effects mainly occur near roads. Road networks interrupt horizontal ecological flows, alter landscape spatial pattern, and therefore inhibit important interior species. Thus, road density and network structure are informative landscape ecology assays. Australia has huge road-reserve networks of native vegetation, whereas the Dutch have tunnels and overpasses perforating road barriers to enhance ecological flows. Based on road-effect zones, an estimated 15‐20% of the United States is ecologically impacted by roads.

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Roads and their major ecological effects

Fluvial Processes in Geomorphology

M AORPHOLOGISTS have accumulated a wealth of observations and conclusions on the development of coasts and shore lines and on the sculpturing work of rivers above tide level, but the landform that owes its configuration to the combined effects of tidal action and river flow, the estuary, has been somewhat neglected-perhaps because its origin has seemed too fluviatile to coastal morphologists and too marine to those concerned with river work. This may explain why the particular geomorphological form discussed here has escaped scrutiny for so long a time, in spite of its prominence and wide occurrence. Past investigators' of meandering tidal streams were mostly concerned with the question whether rivers at tide level can or cannot meander actively; they did not study the form differences between river meanders above tide and meanders shaped with the participation of periodically reversing tidal currents. Numerous estuaries around Chesapeake Bay possess meanders of the latter origin. A check of topographic maps revealed their presence also in other parts of the Atlantic Coastal Plain of the United States, and even on coasts of other continents. These meanders are sufficiently different2 from other meander forms, and at the same time sufficiently frequent, to be classified as representatives of a new meander type, the estuarine meander. The present paper attempts to describe the major characteristics of this meander type.

Frank Ahnert

Papers

Fluvial Processes in Geomorphology

Estuarine Meanders in the Chesapeake Bay Area

L'Epiderme De la Terre: Esquisse d'une geomorphologie appliquee