SI-2A (12/1/04 version)
Environmental Targets And Priorities, Stream Integrity, Stream/Watershed Components: Stream Morphology

Introduction

The influence of stream morphology on overall stream health is fundamental. Therefore, improving stream integrity must start with an assessment of existing channel morphology conditions. The first step in the process is to develop an understanding of the critical morphology issues, not just at one location, or sporadically, but of the entire drainage network. While extensive measurement or classification of streams1 may be useful, it is most important to ascertain the key aspects responsible for lack of integrity or sustainable integrity. The "indicators of stream channel sustainability" below describe those aspects of stream morphology that are most fundamentally important for overall stream health and options to assess their status.

1 "Chapter 3, Rosgen Stream-Classification System/Channel Assessment and Validation Procedures" in the following pdf http://www.bae.ncsu.edu/programs/extension/wqg/sri/stream_rest_guidebook/sr_guidebook.pdf

Once the indicator data below has been collected, it can be used to direct management or corrective actions. Measuring stream morphology generally means measuring the channel shape and form, while determining management or rehabilitation options typically requires comparing "as is" conditions to target conditions.

Indicators of Sustainable Stream Channels

While channel morphology strongly influences the integrity of stream resources, not all aspects of channel morphology are equally important. The following are hierarchical indicators for characteristics of stream morphology. The indicators are in order of importance for evaluating the physical condition of streams, their existing condition, and/or proposed modifications and restoration.

VERTICAL CHANNEL STABILITY

Vertically stable streams are in a dynamic equilibrium, with a balance between bed material transport and supply. Vertical instability can be either degradation (down-cutting) or aggradation (filling up).

Degradation is by far the most common vertical instability threat to Ohio streams. It is caused by increased bed load transport, the result of increasing runoff (i.e. urbanization), shortening stream length, and restricting floodplains. When a watershed is "hardened" by impervious surfaces such as asphalt and rooftops, or when dams trap bed load material, the reduced bed load supply causes down-cutting. Down-cutting can jeopardize infrastructure such as buried utility lines, and bridge abutments.

If more bed material is washed away than is supplied, incision results, which lead to channels becoming entrenched having less access to floodplains, and less interaction with riparian areas. Entrenched channels are characterized by low stream quality and long-term instability.

Even high quality streams will continue to decline and their desired characteristics diminished if they are not vertically stable. Vertical stability is a necessary element for every other desired characteristic of channel morphology.

Assessment

A variety of approaches exist for evaluating vertical stability. They range from very sophisticated sediment transport models, to bed material threshold of motion criteria, to simple subjective observation. Sediment transport models are appropriate where existing conditions are already vertically unstable and the consequence of failure severe. Subjective observation is appropriate in vertically stable channels where the existing conditions can be improved upon. For example, vertical stability need not be a concern in low energy streams, where drainage ditches (straight and entrenched) can gain floodplain and stream length.

FLOODPLAIN FORM

The most significant aspect of morphology for the long-term quality of a stream is floodplain form. Vertical stability is the first objective primarily because down-cutting of the channel bed causes a relative change in the form and function of the floodplain.

The vast majority of natural streams have extensive floodplains that are saturated or inundated several times a year. This is characteristic of streams with slopes less then 2%. The average stream slope in Ohio is 0.3%. The stability of steeper streams is also dependent on appropriate floodplain widths, generally narrower with increasing slope.

When floodplains are extensive, low and frequently wet streams have a tremendous ability to assimilate pollutants. Also, they are more resilient, more resistant to watershed impacts and pollutant loads.

Furthermore, while channels have a capacity to recover over time, the natural formation of new floodplain is the aspect of stream evolution that takes the most time, and causes impacts downstream. When encroachment occurs, prospects of floodplain recovery diminish. Likewise, without human intervention, lower energy channels do not easily recover active floodplains.

Assessment

The quality of floodplain form is first an issue of elevation; and second, an issue of width. Ideally, floodplain elevation is at and below the "bankfull stage" and allows the ground surface to become saturated or inundated several times a year. Floodplain elevation is quantified as ratios of the relative height of the floodplain to the channel depth, frequency of inundation, or relative depth of a particular recurrence interval flow.

Defining adequate floodplain width is justifiably nebulous. One potential target condition is pre-disturbance conditions. Another is the width of the meander pattern past, present or projected. For most streams it is simply a matter of the more the better. However, floodplain immediately adjacent to the channel is most critical with diminishing importance with increasing distance. Because floodplain width comes at a cost, and frequently conflicts with other land uses, a delineation of ranges of floodplain width is provided.

The adequacy of floodplain (for streams that naturally have floodplain) is defined for four ranges by the following three descriptive reference points. Floodplain width dimensions defined here are in multiples of the bankfull channel width and refer to the width that is saturated or inundated by bankfull flow. It is generally the width of flow just above the bankfull stage.

Three times the bankfull width is the bare minimum. Below this threshold streams are characterized by poor quality, lateral instability, degraded habitat and minimal or negative watershed benefit.

Five times the bankfull width is frequently associated with fairly good streams. Clear ecological benefits are associated with floodplains of this width. Flood hydraulics and sediment transport exhibit a break at about this floodplain width, with bedload transport increasing at a faster rate below this point. Lastly, common meander pattern beltwidths start to become restricted below this floodplain width.

Ten times the bankfull width and greater is typical of the highest quality streams, pristine conditions and streams that provide considerable downstream benefit by their assimilative capacity and hydrologic effect.

A number of indications and arguments point to 10 times the channel width as a general threshold below which stream quality is limited. High quality E-4 stream types naturally have very wide floodplains (57 times the channel width is the average of Rosgen's Classification references, Rosgen, 1994). Below this threshold, the floodplain width appears to begin limiting stream quality, changing the hydraulics and interactions between the channel and floodplain (Ward, 2000).

The effects are gradual until the floodplain is reduced to a width somewhere around 5 times the channel width. Below this point, floodplain width becomes a significant limiting factor until the lowest threshold is reached somewhere around 3 times the channel width. Below this one cannot reliably expect a floodplain to exist, benefits are not realized and the reach becomes a source of problems locally and downstream.

CHANNEL FORM

The form of the bankfull channel is its cross sectional dimension, meander pattern and bed form. While channel form is commonly emphasized in design, its relative importance follows vertical stability and floodplain form.

Assessment

A number of channel form assessment and design approaches have been developed. Using all available approaches may be appropriate for intensive restoration of particularly high quality, large or unstable streams. For less critical projects and lower quality vertically stable streams more generic channel design may be adequate. Some projects may even involve only floodplain construction with no work done to the bankfull channel.

Commonly used sources of assessment and design information include:

  • Regime equations (Empirical Design): The large databases of channel geometry have a statistical advantage in that they are based on many streams and rivers. Their disadvantage is that they describe typical values within large acceptable ranges without defining specific conditions.
  • The Rosgen Classification of Natural Streams: Rosgen's classification of natural streams allows a level of refinement, where typical channel geometry is provided for stable channels in a range of valley conditions. Perhaps even more valuable is the Rosgen Classification's geometries that are characteristically unstable.
  • Regional curves: Regional curves provide another type of refinement. Similar to the Williams equations, they generally provide only a description of a typical stream without accounting for the variability in channel character. However, they do describe channels typical of a region. Typically, only three values provided bankfull cross sectional area, width and mean depth (actually only two variables since mean depth is defined as area/width). These are useful for size and width to depth proportions both having regional variability.

    For an example of a watershed specific regional curve click here (hyperlink to OU project).

  • Measured local reference channel (Analog Design): A reference channel reach near the reach to be designed has the best potential for defining channel variability and character resulting from local climate and geology. Additionally, there is no end to the detail that may be gleaned from a local reference. However, the likelihood of a local reach having been surveyed and published is typically low. Perhaps the most valuable (and time consuming) design task is surveying a local reference reach, its longitudinal profile, representative cross sections, bed materials and to a lesser extend its meander pattern. The weaknesses of measured local or on-site channel geometry is that a quality channel might not exist. It is also a small sample and thus should be used in conjunction with the previously mentioned three sources for determining stable channel form.