We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi-quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrockor boulders on the streambed.

The median grain diameter (D50) is increased slightly by dam construction,but D50 remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of thedam has therefore only influenced the fraction of finer-grained sediment on the bed, and has not caused other measurable changes in fluvial morphology.

The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well-vegetated and cohesive banks, and low rates of bed material supply and transport.

If the dams of our study are removed, we argue that long-term changes (those that remain after a period of transient adjustment)will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal instreams similar to those of our study area should not result in significant long-term geomorphic changes.

The flow regime of the gravel-bedded river North Tyne has been regulated by the Kielder reservoir for the past 12 years; for the past nine years, regulation has been dominated by hydropower generation. Diurnal stage fluctuations of up to 0.6m are experienced during periods of peak hydropower flows. The main morphological and sedimentological impacts of this regulation are identified and physical explanations provided for the observed adjustments. The main morphological adjustments are identified as the degradation of riffle spawning grounds, the development of fine ssediment berms along channel margins, the aggradation of pools, vegetation of former gravel shoals and the growth of tributary confluence bars. Sedimentological adjustments are subtle and are characterized by higher percentages of fines within spawning gravels, coarsening of surface gravels and the development of a stable, strong bed fabric. The physical explanations for these adjustments relate to changes in the sediment transport regime controlled by the hydraulics associated with the pool-riffle swquence during hydropower generation

Modifications of lower watersheds such as water abstraction, channel modification, land-use changes, nutrient enrichment, and toxic discharge can set off a cascade of events upstream that are often overlooked. This oversight is of particular concern since most rivers are altered by humans in their lower drainages and most published ecological investigations of lotic systems have focused on headwater streams. Factors contributing to ecological processes or biophysical legacies in upper watersheds often go unacknowledged because they occur at disparate geographic locations downstream (e.g., gravel mining, water abstraction, dams) with significant lag times. This paper considers examples of how alterations to streams and rivers in their lower reaches can produce biophysical legacies in upstream reaches on levels from genes to ecosystems. Examples include: 1) genetic- and species-level changes, such as reduced genetic flow and variation in isolated upstream populations; 2) populations-and community-level changes that occur when degraded downstream areas act as population "sinks" for "source" populations of native species upstream or, conversely, as "source" populations of exotic species that migrate upstream; and 3) ecosystem-and landscape-level changes (e.g., nutrient cycling, primary productivity, regional patterns of biodiversity) that can occur in headwater systems as a result of downstream habitat deterioration and hydrologic modifications. Finally, a case study from my own research illustrates the importance of careful consideration of downstream-upstream linkages in formulating research questions, designing experiments, making predictions, and interpreting results. The effects of dams and associated water abstraction in lowland streams of Puerto Rico has forced my colleagues and me to re-evaluate the results of ecological research that we have conducted in highland streams over the past decade and to redirect our research that we have conducted in highland streams over the past decade and to redirect our research to consider downstream-upstream linkages.

The goal of this article is to illustrate a geomorphically based approach to understanding and reducing some of the cataclysmic effects of dams on downstream riverine ecosystems. Additionally, we hope to provoke debate on the merits of different approaches to river conservation. We suggest that geomorphic studies directed at ecologically significant features of river morphology and hydraulics may sometimes be more valuable in short-term evaluations associated with the environmental assessment of new dams or the relicensing of existing dams. We believe that more often than has been realized, geomorphological changes are the key to understanding the long-term ecological consequences of dams and other stream disturbances. We also have some reservations about the efficacyof biological research in short-term studies to lead to successful management plans. The biological research necessary to develop a management plans is substantial--often a multiyear undertaking--and may not lead to a clear understanding of improtant features affecting the ecosystem.

Healthy fish populations are dependent on streamflow regimes that protect the ecological integrity of their habitat. Fish habitats are the consequence of linkage among the stream, floodplain, riparian, and upland zones, and watershed geography. Fluvial-geomorphic processes form and control fish habitat. Because of this, multiple in-channel and out-of-channel flows are needed to maintain these processes. We present a conceptual methodology for measuring tour types of streamflow regimes: instream flows, channel maintenance flows, riparian maintenance flows, and valley maintenance flows. The combination of these four streamflow types is designed to protect fish and their habitat. Using a case study of the Salmon River near Whitebird, Idaho, we demonstrate how the methodology could be used to develop a multiple flow recommendation.

The Mississippi River is the hardest working river in America: a central artery for commerce, a stormwater management system for the two-thirds of the nation, the central flyway for 40% of the nation's migratory waterfowl. Each of the river's distinct forms of habitat is disappearing: backwater marshes dominated by emergent plants are filling in or, alternatively, becoming open, lifeless turbid waters; floodplain lakes have filled with silt; aquatic plants are not replaced because perpetually turbid waters block light penetration; sediment buries mussel beds and deepwater pools' islands erode, eliminating mast-producing forests. High water tables undermine floodplain forests, which lack higher ground to replace themselves. Restrictions of fish movement by the dams makes the decline of habitat in a particular pool more significant by blocking fish access to habitats in another pool. These problems are exacerbated by current river uses, and by past and present land uses that have altered basinwide hydrology and accelerated the rate at which sediment enters the river. Sediments and nutrients enter the river at unsustainable rates due to past and present land use practices that increase erosion and eliminate wetlands and stream-side buffers. Commercial and recreational vessels resuspend sediments in the water column, blocking light penetration and contributing to the loss of backwaters. Even in its reduced for, the Upper Mississippi represents the last piece of Midwestern America's Great Rivers, supporting migrating waterfowl, endangered mussel species and the most ancient lineage of fish in North America. Whether this system continues to survive and flourish depends on whether dynamic river forces can be sufficiently restored to make the river system self-sustaining. Preserving and restoring the Upper Mississippi and Illinois rivers requires three types of actions: 1. Recreate dynamic river forces to achieve self-sustaining habitat restoration 2. Minimize the operational impacts of the navigation system 3. Achieve no net increase in sediment by 2010

River regulation imposes primary changes on flow and sediment transfer, the principal factors governing the alluvial channel regime. In this study, the effect of flow regulation is isolated from sediment delivery. Peace River was regulated in 1967 for hydropower. The gravel-bed reach immediately downstream from the dam has become stable. Gravel accumulates at major tributary junctions, so the river profile is becoming stepped. Further downstream, the river has a sand bed. It can still transport sand, so morphological changes along the channel include both aggradation and channel narrowing by lateral aggradation. In the gravel-bed Kemano River, the addition of water by diversion from another river caused degradation when additional bed material was entrained below the inflow point. However, the effect became evident only after many years, when a competent flood occurred. The short-term response was channel widening. The time-scale for the response depends on the size of the river and the nature and severity of regulation. In both rivers, significant adjustment will require centuries and will intimately involve the riparian forest.

Water resource developments which deplete the quantity or reduce the range of streamflows usually have a number of unintended effects on the channel downstream, including loss of channel capacity, loss of aquatic and riparian habitat, and channel instabilities. A method for identifying a flow regime sufficient to maintain desired stream characteristics, while permitting significant development, would have great practical value. over the past decade, important advances have been made in our understanding of fluvial processes in gravel-bed streams. Using these advances as a basis, one can outline a method for determining channel maintenance flows for gravel-bed streams typical to the western United States. A common characteristic of gravel-bed streams is that bed particles are transported only about 5-10 percent of the time during the highest flows, and, even then, at a very low rate. Although occasional motion of bed particles begins at a discharge as small as 60 percent of the bankfull value, general motion of the bed surface is exceedingly rare. The proposed method relies on an appropriate bedload transport function and specific reach of discharge in the pre- and post-regulation regimes. Evaluation of possible flow regimes indicates that bankfull channel capacity can be maintained in its pre-regulation condition where as much as 60 percent of the natural flows are diverted.