Introduction

	Aerial photograph of tidal Dunstan River, Scarbourough, Maine. Photo credit: NOAA.

Rivers are magical landscape features, always moving, always changing from their headwaters as small trickling brooks until reaching their end at the sea. Streams, rivers and other flowing waters are collectively known as lotic waters.  A river's form generally results from its adjustment to the magnitude of water flow and underlying soils, controlling bedrock, and other materials produced within its source watershed (Rosgen 1996). Channel bed materials, watershed topography, valley morphologic features, bedrock structure, and long-term erosion and sediment deposition processes also influence the physical characteristics of rivers. A relationship and balance between river discharge, sediment supply, sediment particle size, and river slope dictate dynamic river morphology (Lane 1955). Modifying one or more of these factors will cause an adjustment in form and/or process to a new set of conditions.  These principles apply to the smallest of brooks and streams to the largest rivers including tidal freshwater rivers to their downstream boundary with estuaries.

Hydrology and Geomorphology

	Example of a third order river - Eight Mile River, East Haddam, Connecticut. Photo credit: J. Turek, NOAA.

Recognizing that broad-scale, regional landscape features influence local-scale stream corridor pattern and morphology is a key principle of river geomorphology.  To describe the hierarchy of stream size, river ecologists often describe rivers and streams size in terms of stream order (Horton 1945; Strahler 1957). First-order streams are the smallest possible streams, have no tributaries, and generally form in steeper landscapes. These streams typically have a comparatively high gradient.  First-order streams may be ephemeral - flowing for only very brief periods, intermittent - flowing seasonally with normal precipitation, or perennial - sustaining year-round discharge with normal weather conditions. Most higher-order streams are perennial but can also occur as intermittent or ephemeral systems. Second-order streams form when two first-order streams join. When two second-order streams meet, a third-order river is formed. This systematic classification continues with fifth-, sixth-, and higher-order rivers forming. Rosgen (1994) developed a stream classification based on geomorphic and hydrologic features, and has become increasingly used by river restoration practitioners. Other stream classifications based on channel/valley form and/or process have also been developed (Montgomery and Buffington 1997; Pfankuch 1975). Benda et al. (2004) have developed a network dynamics hypothesis that suggests the spatial arrangement of tributaries in river networks interact with watershed processes to influence patterns of geomorphology and habitat variation.

Studying, assessing and restoring rivers require an understanding of hydrology and other primary riverine features. The hydrology of a watershed includes base flows, discharge, and storm flows.  A hydrograph of a stream during a storm event depicts discharge, usually measured in cubic feet per second (cfs).  The hydrograph begins as base flow, then with increasing precipitation, rising to a peak, storm flow rate, before receding back to base flow conditions over time. Watershed characteristics dictate flow conditions and the shape of the hydrograph and the steepness of peak discharge.

Stream morphology determines how a stream reacts to storms and other events. Bankfull discharge is the flow of a stream or river at which water levels reach top of banks and begin to spill over into the floodplain or lands adjacent to the stream or river that receives floodwaters and sediment.  Floodplains develop as a result of long-term valley formation through soil/sediment erosion and sediment deposition. Low-order, high-gradient tributary streams typically have steep-sloped valley flanks with non-existent or narrow floodplains. Vertical erosion, termed downcutting, is the dominant process in these headwater areas. Channel alignment of high-gradient, low-order streams within confined valleys is straighter in comparison to the meandering channel pattern of low-gradient, high-order rivers with broad floodplains. Floodplains become better developed in the down-valley direction through the process of lateral migration, where the stream channel reworks deposited sediment transported from upvalley.  Lateral migration is a process through which the comparatively less energy of the stream is expended by erosion on one bank and deposition on the opposite bank.  This process creates stream meanders, or bends, that are characteristic of many lower gradient streams. The degree of meandering or frequency of channel bends is defined in terms of sinuosity.

Stream dimensions and features are generally described as a function of channel width, measured at the bankfull height.  Stream width in most cases increases downstream and can be approximated by the mathematical square root of the associated discharge (Leopold et al.1964). Bankfull discharge occurs at a specific flow rate for a given river stretch, and typically reoccurs about every 1.5 - 2.5 years, as calculated by hydrologists through flood frequency analysis. The more frequent discharge rates transport the greatest amount of sediment materials over time. Although floods are powerful events resulting in substantial erosion and deposition, they generally occur less frequently. With erosion and depositional processes, natural levees develop along rivers over time as coarse sediments transported in flood waters are deposited along banks where lateral flow velocities quickly dissipate. Scientists and natural resource managers often describe and regulate floodplains in terms of the 100-year floodplain - those lands having a 1 percent chance of flooding in any given year.

In-stream structure both longitudinally along a stream and laterally across a stream influences stream ecology and functioning. The location of the thalweg, pools, riffles, and runs define stream channel structure and affect the presence, abundance and distribution of aquatic biota. Pools are deeper, slower stretches of rivers where finer-grained sediments and organic matter accumulate and fish and other biota often seek refuge. Pools often form along the thalweg near the outside bank along channel bends.  Riffles are reaches characterized by fast-flowing, generally shallow waters with boulder, cobble, and gravel substrates where the gradient is steeper than reaches immediately upstream (typically a run) or downstream (often a pool). Riffle features form between two bends where the thalweg crosses from one side of the channel to the other. Runs are reaches that develop at the end of pools as water depth decreases and flows gradually increase over fine-grained substrates before passing on to the next riffle or pool feature.

Stream hydrology is also affected by the riparian habitats bordering these flowing waters. The plant communities including trees, shrubs and herbaceous and other ground cover plants that characterize riparian habitats often form dense canopies over streams and rivers and thus influence the amount of precipitation that falls directly to streams and rivers. Riparian plants also uptake rainfall and runoff that percolates into the ground, and release water through evapotranspiration.  Many of the nutrients that make their way into streams originate in riparian habitats. Riparian habitats are defined as wetlands based on the presence of temporary to seasonal surface waters or near surface groundwater, as well as the plant species that are adapted to living in hydric soils.

Disturbance and Equilibrium

	Low flow conditions exposing scoured bank, Norwalk River, Wilton, Connecticut. Photo credit: J. Turek, NOAA.

Physical disturbances to rivers and streams such as channelization (channel straightening); removal of riparian vegetation and in-stream woody debris and snags; and alteration of normal water flows and sediment transport due to dams and undersized or poorly functioning culverts affect channel morphology and stream stability. Natural or human disturbances to streams and rivers thus affect both the structure and ecological functions of these systems.

East Coast Rivers and Streams

	Upper Connecticut River, Fairlee, Vermont. Photo credit: J. Turek, NOAA.

East Coast rivers and streams are highly diverse in size, length, physical and chemical condition, and in the plant and animal communities that inhabit instream, wetland riparian and upland watershed habitats. The following are some general examples of East Coast systems for readers to recognize: