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similar to the one shown in Figure S. Such streams are usually relatively stable between periods of high flow. Care should be taken not to overlap the high flow period during the data collection pro` cess. If a point of interest is located just upstream from its juncture with a larger stream, the flow in the larger stream may control the stage in the smaller stream. This phenomenon is called "variable backwater," because at some times of the year the backwater is present, and at other times it is either absent or- of much reduced extent in the tributary stream. The simplest solution to the problem of variable backwater is to avoid areas where they occur. If this is not possible, a stage-discharge relationship can be established during the period of the year when the backwater is not present or is of reduced extent. Assessment of the backwater during high flow periods should be tied to the stage-discharge relationship of the main stem river. Other factors commonly influencing the stage-discharge relationship include aquatic vegetation and ice. The effect of aquatic vegetation is to increase the roughness and to decrease the cross-sectional area of the channel. The result is an increase in stage for a given discharge in comparison to the channel without aquatic vegetation. In streams with an attached algae, such as Cladophora, the stage-discharge,relationship when the algae is dormant may be much different than while it is growing rapidly. In this situation, it may be necessary to construct two rating curves, one for the growing season and one for the non-growing season. Ice has a complex impact on the stage-discharge relationship. The presence of ice decreases the cross-sectional area, changes the wetted perimeter, and changes the roughness. The types of changes involved. depend on whether the ice is surface-formed (sheet ice) or bottom-formed (anchor ice). It is enerall advisable not to attempt to construct a rating curve, or use the Manni n e uati.o n when i ce i s red sen77 n the stream. For the most part, these generalised limitations should serve as warnings to the field practitioner, not prohibitions. A stage-discharge relationship may be developed under any of the conditions mentioned above. However, if these conditions exist, extra care in study planning and data collection are warranted. LIMITATIONS TO THE USE OF THE MANNING EQUATION From the instream flow assessment standpoint, the greatest advantage in the use of Manning's equation is that only one set of measurements is needed to calibrate the equation. For an agency charged with determining instream flow requirements of many streams, frequently with little time or resources to accomplish the task, this advantage may be extremely attractive. Unfortunately, the same factors giving the advantage are also involved in the limitations to the approach. ZS