As I sat by the edge of the boulder strewn stream, I reflected upon the uniqueness and similarity of every river I have fished. The Credit River below Cataract is not the West Branch of the Ausable River in upper New York State and yet the pocket water, the foam lines, and the trout lies are the same.  I had experienced a similar "dejá vu" while fishing other rivers: portions of the Grand River and Saugeen River remind me of the Beaverkill; portions of the Sydenham River remind me of Flat Creek in Wyoming; portions of the River Dee in Wales remind me of the Maitland River.

What is it about these rivers that on one hand is unique and yet so familiar? How is it that you might never have fished a particular river before and yet you just know where the trout or bass will lie?

Perhaps part of the answer lies in how rivers and streams are formed. It is true that every part of the earth is unique, with a slightly different mix of soils, geology, climate, vegetation and animals. But ultimately there are still universal physical laws that govern the way climate, geology, and topography of an area shape a watershed, the pattern, volume, and characteristics of water flow and the valleys and stream channels found within them.

A wild cowboy from Colorado helped me to understand the underlying patterns of rivers that I had observed over the last 30 years. As well as a cowboy, Dave Rosgen is a flyfisher, hydrologist, and geomorphologist with 30 years of experience in researching and repairing rivers. Dave was suggested as a speaker at a conference on trout stream rehabilitation that I was chairing back in 1990. A mutual friend, Don Duff, from the U.S. Forest Service in Utah said, "You gotta have this guy speak at the conference. He just knows rivers and loves them".

Dave knocked our socks off at the conference. His presentation was like a bombshell for many of us. Besides his comments about the "pin-headed snarfs" that damage rivers and try to make them do what they are not meant to do, he demonstrated that rivers have inherent patterns and forms based upon the slope of their valleys, how tightly confined the streams are within their valleys, the size of soil and rock the streams have to work with, and other factors.

"When the works of man run contrary to the natural tendencies of the river," Dave exclaimed, "the river eventually wins."

One year later I organized and participated in a one week course on streams taught by Dave. During the one week course, my mind was rocked by revelation after revelation about streams and their characteristics. "It was like flashbulbs constantly going off in my head," I explained to a friend a few weeks later. That comment was echoed by both biologists and engineers taking that particular course and several others taught by Dave in subsequent years.

Dave's course enabled me to understand the relationships I had observed about river forms, eroding banks, spawning areas in streams, and the appropriateness of various stream improvement techniques I had tested over the years. Dave was able to explain and demonstrate how and why the traditional approaches to channelization, rip-rapping and river "engineering" almost always fail - "they are contrary to the natural tendencies of the river." In addition, other shocks revolved around a glimmer of understanding of where and why trout and bass used certain habitats in different types of streams.

Dave has been able to develop a classification that helps to define typical stream types. The types range from A to G. The basic stream type patterns are shown in Figure 2.7 and demonstrate certain similar characteristics of slope, width, and depth and sinuosity based upon analysis during the bankfull flows of a river. The classification is based upon research using data from over 450 river systems throughout North America and New Zealand.

The major shaper of the stream channel is a certain volume of flow that has sufficient energy to begin to reshape and adjust the channel. Dr. Luna Leopold, son of Aldo Leopold of Sand County Almanac fame, noted that flood flows that typically occur once every 1.5 - 2 years are the major flows that adjust channels, sort the bottom of rivers, move materials to deepen pools and reshape riffles. This flow is called the "bankfull" flow. He and others coined the phrase, "dynamic equilibrium" to explain that even though the actual location of pools and riffles, bends, and straight portions changed and adjusted after every bankfull flow, the shape and form of a stream appeared to be the same over the years. In this way a stream moves within its valley at a very slow and controlled rate, adjusting for minor variances in flows and sediments. This is a major natural tendency of a healthy river. In this way, given the valley slope, soils, and other controls, the river maintains the appropriate shape and form of channel that is the most efficient to move and store both water and sediment at all flows.

Stream types are determined by a length of channel that has the fitting characteristics for the particular type. Based upon statistical analysis, stream classification requires a minimum of two full wavelengths of stable channel in order to determine the type of stream. Fundamental assessment characteristics include:

• Wavelength is defined as two complete riffle:pool sequences and is usually 10-14 bankfull widths in length. In some larger streams this requires a stable channel length of 0.5 to 1.0 kilometres!
• Slope is measured as the percentage change in gradient from an upstream point to a downstream point along the river channel. For example, a stream with a 2 per cent gradient means that the streams falls 2 metres for every 100 metres of channel.
• Width and depth are often examined as a ratio called width:depth, which is usually measured at a riffle or shallow point in the stream. For example, a stream with a width:depth ratio of 30 means that at the riffles the stream is 30 times as wide as it is deep.
• Sinuosity is also called meandering, the way a stream wanders back and forth down its valley. It is often measured as a ratio of the channel length versus valley length. For example, a stream with a sinuousness of 1.5 means that for every 100 metres of stream valley you have 150 metres of stream channel.

The sediment found on the bottom of streams is there as a result of transport down the channel during high flows. Coarse material in the riffles indicates the sizes of material that is either moved every high flow or only moved by exceptionally high flows. During near bankfull flows, sediment moves along the bottom of the stream as bedload. Depending upon the gradient and form of the stream and its discharge, materials in transport during floods can range in size from silt to large boulders. The mean size of particles in the bottom of a riffle in a stream indicates the size of substrate moved by the bankfull. The sorting of gravels and cobbles in riffles during bankfull discharges is an important process for the "conditioning" of riffles for aquatic insect production and fish spawning. During high flows, the areas of the stream with the greatest erosion are usually in the pools; this is why pools are deep, and the areas of greatest sorting and deposition of substrate are the riffles. Point bars and floodplains are the locations of deposition and storage of finer materials.

Most anglers are familiar with three basic types of streams: pocket water streams; riffle:pool streams; and meadow creeks or spring creeks. These overlap the scientific stream types developed by Dave Rosgen. Pocket water streams fall into the "A", "B", and sometimes "G" stream types. Riffle:pool streams fall into the "C" and "F" and occasionally "G" or lower slope "B" types and meandering meadow and spring creeks fall in the "E" stream type. Typical planview or top:down view and lengthwise side view of pocket water and riffle:pool streams are illustrated in Figures 2.8 and 2.9.

The "A" and "B" type streams are typical mountain or escarpment streams, tumbling down a mountain side in a more or less straight line. In "A" and "B" channels, a step:pool sequence repeats itself every 1-5 bankfull channel widths (depending upon slope). The "A" type streams have steep sides, a bottom of bedrock or boulders, and create high energy pocket water, referred to by hydrologists as step:pool sequences (Figure 2.7). "A" channels also have a fairly narrow profile of width and depth. The "B" channels are also steep, although not as steep as "A" streams and have a "bowl shaped" channel when viewed in cross-section rather than steep slopes (Figures 2.7 and 2.8). These channels can have boulders and cobble and can range from pocket water to modest pools and riffles (eg. the Credit River from below Cataract downstream to below the Forks as in Figure 2.10). Figure 2.8 shows the relatively straight form of a "B" channel along with the boulder steps that create the plunge pools and pocket water below them. The lengthwise cross-section shows a sharply undulating bed as you move downstream. I believe that metre for metre, these streams have more juvenile and adult habitat along the bed than any other stream type. Fish can be found almost anywhere in these streams. However they often do not have the range of habitat required for all stages of the life cycle of trout or bass, and, therefore, in order to be fully exploited and productive, there also needs to be "C" channel reaches available in the same system in order to provide spawning and early nursery habitat for the trout or bass community.

By far the most common river class in southern Ontario is the "C" type channel, specifically the C3 (cobble) or C4 (gravel) channels such as the Saugeen River below Durham, the Credit River above and below Inglewood, the Maitland River below Wingham, and the Grand River below Elora Gorge all the way to Brantford (see Figure 2.9 and 2.11). These streams are the classic riffle:pool streams common to all areas of the world, where you find a modestly wide valley, good soils, and modest valley and stream gradient. The "C" channels along with "E", "F" and "G" channels, have riffle and pool sequences that occur every 5-7 bankfull channel widths as you move down the stream. These are the streams that meander in a mild fashion through their landscape. The outside bends have the pools and steepish sides, often with log jams and protruding root wads, the inside bends have point bars with sorted gravels and sands, plus a shallow floodplain behind them. The riffles are found half way between the pools. Figure 2.9 shows the gentle meandering form of the riffle:pool stream along with the characteristic logjams and wood debris at the bends and at the edges of the channel. The bed of these streams undulates in a more gentle manner, with a wider spacing between deep sections (pools) than found in the high energy, high gradient A and B streams. Pools may be wider spaced but they are usually much deeper than in "A" and "B" channels. A healthy "C" channel has all the important requirements for trout as long as temperatures are appropriate. This stream type is also ideal for bass, as long as the stream is large enough. Research in the United States, for example, found that a stream had to be, on the average, at least 10 metres wide before it had sufficient size, depth and complexity to hold bass year-round.

Although "C" channels do not have as much "usable" habitat throughout the channel, the pools often make up for it. The depth, size, and complexity of habitat within a pool, especially one with logjams, root wads, and undercuts, can hold surprising numbers of juvenile and adult fish. For example, studies done on the Credit River have found that every healthy, stable pool on the Credit with a log-jam appears to have at least 6 to 10 kilograms of trout in it. This weight of trout often equates to 30 to 50 trout ranging in size from 20 - 50 cm.

Many of the most common spring creeks in Ontario, such as the Sydenham upstream of Chatsworth, as well as the more interesting ones in Montana such as O'Dell Creek, and Wyoming's Flat Creek, are called "E" channels (see Figure 2.7). These streams have extreme meanders that seem to loop back and forth with wild abandon. They are extremely deep for their width, often with a width-depth ratio of less than one. For example, upper Willow Creek, north of Barrie, is only approximately 5 - 6 metres wide but is 3-10 metres deep! I am certain that many of us know of these creeks. They are the ones that look so small but when we step into them we go over our waders! These types of streams occur in wide, shallow valleys often called "water meadows." They have very low gradients. The sinuousness and character of these streams are controlled completely by the vegetation along the banks. This control also creates the incredible undercut banks found in these streams. These streams have almost as much usable linear habitat for fish as the "B" channel and they have more volume of habitat because of their greater depth. In one structurally healthy section of "E" channel on the Sydenham River, MNR sampled 84 brown trout of 30-45cm in 50 metres of stream!

A few of the other channel forms are less common in Ontario, although some are present. The Colorado River flowing through the Grand Canyon is classified as an "F" channel. These are streams with a low gradient, mild sinuosity and relatively flat channel cross-section. Ontario has a few "F" type channels, the most familiar being the Grand River through Elora Gorge.

Any particular watershed anywhere in the world, depending upon its topography, geology, soils, climate, and vegetation will likely have a variety of stream types found within it. For example, the Credit River has portions of "B", "C", "E", and "G" type reaches, although the most common type is a "C" channel. A representative C4 stream with the typical gravel bed and meandering riffle:pool channel is found between Inglewood and Cheltenham. Above the Meadow section in the Forks Provincial Park there is a B3 channel with the typical step:pool, pocket water cobble bed stream.

It is the characteristics of the watershed that control the volume, timing and duration of high flow patterns that dictate the physical and biological functions of our streams. These flow patterns, when combined with a watershed's soil characteristics, valley slopes, and streamside or riparian vegetation, generate the dynamically stable channel forms we see and the physical and biological functions they perform. If the characteristics of the watershed and valley stay the same over time, stream sections retain their form and character. If, through landuse change, the watershed's water flow characteristics and sediment supply changes, the stream will begin to adjust. If the process of change is very slow, the rate of change will be gradual and controlled naturally. If the changes are rapid, such as massive urban development without significant sediment and stormwater control, the stream adjustments will occur so rapidly that the stream's form will become unstable, and erosion, flooding, and channel degradation will occur. As this happens, fish habitat will be lost, fish communities will change then collapse, and water quality will severely deteriorate. We have many examples of these processes in southern Ontario.

Many of our streams in Ontario are in the process of transition and adjustment from one type of stream to another so often that it is difficult to determine what stream type the reach was, and to what type, within the limits of the valley slope and soil, it is trying to evolve into. However, the direction of the evolution of the stream can be determined through careful analysis and measurement of the stream's geometry, slope, new flow, and sediment conditions. This information can then be used with the classification system to determine the dynamically stable type the reach should be, given the new conditions.

Do fish care what type of channel they live within? I doubt it. However, fish such as trout, bass, pike, walleye, and the forage fish and bugs they feed on, take advantage of the features of healthy, stable stream channels exemplified by the Rosgen stream types. Fish use the various channel types in different locations within a watershed for various parts of their life cycles. There are numerous characteristics within stream channels that are important for fish and fish communities. The patterns and stability of riffle:pools or step:pools create habitat conditions for shelter, food, space, and reproduction. The quality of pool area defined by depth, extent, location, and complexity provides shelter, feeding, and overwintering for many species of fish including: salmon, trout, bass, pike, walleye, muskie, sturgeon, suckers, and minnows. Riffle areas provide shelter and feeding habitat for some species such as sculpins, darters, dace, and northern hog suckers. For other species such as trout, salmon, and walleye, riffles provide only feeding and reproductive habitat.

All species of salmon and true trout (brook trout are a char and have slightly different needs) require rivers with a coarse substrate; well defined and sequenced riffles and pools or step:pools; low levels of fine silts and sands in the riffle substrates (spawning is poor when too much fine sediment clogs the spaces between gravel in a stream bed); deep pools, ideally with log jams and/or undercut banks used by juvenile and adults; and a complex, shallow edge, composed of mildly irregular shoreline, ideally with small amounts of wood debris for fry.

Other species, such as smallmouth bass, require well developed pools with a good depth and complexity, small side channels or scalloped margins and riffles and runs composed of coarse rock and boulder. Some species of fish are primarily pool dwelling (eg. white suckers, coho salmon fry, etc.) while others are primarily riffle dwelling (eg. darters, Atlantic salmon and rainbow trout juveniles, and hog suckers). In all cases, fish communities require channels that are dynamically stable and functional. The use of channels by aquatic bugs is similar to that of fish species. The key concept is that the life forms that reside within streams require a channel/valley system and surrounding watershed that are healthy and retain their functional characteristics.

Anglers understand the importance of wood in streams. These pieces of tree limbs, branches, trunks and roots provide extremely important shelter areas for fish. Log-jams on an outside bend or root wads adjacent to a run spell out BIG FISH to canny anglers. Smaller bits of twigs and branches along the margins of riffles are extremely important areas for little fish and fry of trout and bass. The way that woody debris locally modifies the characteristics of stream channels is extremely important to fish. It aids in pool formation, provides structural escape cover, and is an important roughness element that influences channel features not only within the main channel of a river but in its side channels, off-channels and tributary systems. The roughness characteristics of woody material in the channel and floodplain system creates a high level of habitat complexity and stability.

In high gradient "A" channels with slopes greater than 2 per cent, woody debris forms debris dams that act to modify sediment discharge, creating gravel storage areas, stable step:pools, and that enhance side channel development. This roughness function is very important in modifying stream energy, creating habitat for fish in these rivers, and influencing structural patterns for some fish species. In "C" channels, woody debris on the outside bends of channels enhances pool formation, reducing width:depth ratios and creating better habitat for fish (Figure 2.9).  Woody debris in the margins of the stream creates habitat for fish fry and older juveniles. It was found to be an important attribute of rivers for fish habitat (eg. trout), no matter what the size or order of the stream.

In low gradient streams, too much wood debris in the channel can actually destabilize and degrade a functional channel. Subtle changes in roughness of a channel can have major implications on the ability of a stream to mobilize or transport sediment, thereby modifying or altering the habitat of the streambed for bugs and fish reproduction. Too much wood throughout a low gradient channel can actually reduce the stream's ability to move its sediment. This can lead to too much sediment storage in the channel and, ultimately, to accelerated bank erosion. This destabilizes the channel and reduces fish habitat. In the past, many people believed that all wood in streams was bad because it slowed down the flood flows of rivers and caused flooding and erosion. Wood debris that is along a bank or on the outside bend of a river is often not the problem in these circumstances; rather, excessive flooding and erosion occur in rivers that do not have the right width:depth ratio for the type of stream they are. A channel that is too wide will not efficiently convey sediment and water during floods, and will, over time, fill with sediment. This will then make bank erosion and overbank flooding worse and worse. Instead of managing for healthy channel characteristics, the past, and often present solution is to remove all wood from the channel and then to widen the channel even more! This simply worsens and accelerates any previous mistakes.

Most species of fish exploit the various physical characteristics of stable/dynamic channels for reproduction and nursery habitat. Many of the salmons and trouts bury their eggs in riffles that are actively sorted by the annual bankfull flows (especially those of 1-5 order streams). The sorting of the riffle substrate, especially at the transition point between pool and riffle, reduces the concentration of fines in spaces between the gravel and cobble of the substrate. These fines can accumulate between bankfull flows. Fish such as walleye, that are broadcast spawners, prefer to spawn in cobble riffles which also likely benefit from resorting or the flushing of fines that occurs after major flow events. When stream substrates become packed with fine sediments, reproductive success of salmonids and likely many other species of fish can become severely impaired. The packing of fines into the substrate of streams is usually the result of surplus discharges of sediment. The source is surrounding lands or bank failures that discharge sediment in excess of the natural stream sediment budget.

Stable banks and healthy bankside or riparian vegetation along the river channel are interconnected. Well vegetated stream banks exert a fundamental control on channel form and shape. Although this control may vary depending on the size of stream, vegetation in the riparian corridor helps to regulate and modify channel characteristics as well as other parameters such as temperature. There are high levels of roughness exhibited by banks containing rooted vegetation. These roughness characteristics modify flow patterns adjacent to the banks, thereby reducing bank erosion and increasing pool scour during floods. The increased resistance of the bank to scour results in a decrease in width:depth ratio of the stream, deepened pools, reduction of total sediment eroded into the channel system and aids in the movement of materials already in suspension or mobilized as bedload. I think that river anglers have likely noted the importance of well vegetated banks. Streams having lush vegetation of shrubs and grasses or trees and shrubs have better pools and more fish cover in them than stream sections with thinly grassed banks and active bank erosion.

In headwater systems, rooted vegetation in the riparian zone and along the bank is likely the single most important control on shape and pattern of the channel and the quality of fish habitat in the form of logjams and woody debris. In larger portions of the watershed, however, the volume of water has more energy to exert some control on the river's shape and pattern and general in-channel characteristics. In larger rivers, the size and complexity of pools and side margins becomes most important for fish, and vegetation becomes extremely important in the management and health of the floodplain.

Vegetation is the strength and weakness of streams in headwater areas, and especially E type channels. Loss of deeply rooted vegetation along these streams will cause massive bank erosion and channel adjustment, ultimately creating a different type of stream channel. If the process is relatively slow the channel will adjust to a "C" type channel; if it is fast, the channel will become a "D" or braided channel. "D" channels are highly unstable systems and are extremely poor environments for fish, bugs, or people. At the same time vegetation is also extremely important to big rivers. Healthy, densely rooted vegetation helps to hold the banks of larger streams, is an important element of floodplains, provides woody material for fish habitat and pool enhancement, and the leaves and needles provide an important organic food source for aquatic bugs. The elevation of the baseflow channel provides the control on the shallow watertable under the banks and riparian zone. This, in turn, ensures the health of the vegetation. All elements of the system therefore rely upon one another.

The shape, form and characteristics of rivers are not accidents. These forms are based upon physics and physical laws are universal. It is when we, in our arrogance, believe that we can alter channel characteristics with impunity and "go against the nature of the river," that, as Dave Rosgen says, "The river will eventually win; it will kick ass; to our great loss and pain."

We need to be friends of the river, and, if we are, it will be our friend.