In Depth Aquatic AIM Critical Concepts


Welcome to the in-depth video on Aquatic AIM critical concepts. This video discusses the major concepts within the Aquatic AIM protocol that are essential to successfully sampling a site and how they are related to the protocol. The topics in this video are also covered in TR1735-2, pages 8 through 11, within Appendix B on pages 81 through 83, and in Appendix D on pages 93 through 95. The critical concepts covered in this video are bankfull, floodplain, and scour line. It is absolutely critical to correctly identify bankfull in the field. Bankfull is the height and width on the streambank at which the stream fills the channel and begins to overflow onto the floodplain. Bankfull height is determined by the water height that corresponds with channel forming flow, which occurs every 1.5 years. Bankfull is the most important concept to identify correctly at sampling sites. Reach length and many other measurements are determined by bankfull height and width. Bankfull height and width is measured at all eleven main transects within the reach. When identifying bankfull in the field, there are several characteristics that should be used for correct identification. It is important to note that not all of these characteristics will be present at every stream, a combination of clues should be used to correctly identify bankfull throughout the sample reach. Bankfull is at a relatively consistent elevation in stream and river systems, so when arriving at a sample location, it is a good idea to walk up and down the stream and look out for bankfull indicators that occur around the same elevation within the sample area. One of the most obvious indicators of bankfull is a change in streambank slope. In many cases, such as this one, bankfull is located at the spilling point where water would leave the stream channel and start to flow onto the floodplain. This is where the slope of the bank goes from very steep to nearly flat. In this case, floodplain and bankfull will be at the same elevation In many systems, such as those that are incised or are in a V shaped valley, bankfull is not related to the floodplain. That’s when other indicators should be used to identify it. One of the other bankfull indicators is a change in substrate size. Substrate often becomes finer and less well-sorted at and above bankfull. In some streams, bankfull can also be identified by a change in vegetation. This can look very different in each system, but usually presents as either a transition of grassy and herbaceous vegetation to woody vegetation, a transition of wetland shrubs, such as willow and dogwood, to trees, or is the lowest elevation of established cottonwood and alder trees. Point bars, or gravel bars, that are usually on the inside of a meander in a stream can be an indicator of bankfull. The highest point of these bars can sometimes indicate the lowest possible height of bankfull. Bankfull can also be identified by the staining or scouring of rocks, especially in big river systems. Finally, bankfull will usually be just above the elevation of undercut banks. There are many lines of evidence that can be used to identify bankfull, and in most systems a combination of these indicators must be used to identify bankfull throughout the reach. The list of indicators can be found in the protocol TR-1735-2 on page 9. It is helpful to remember that bankfull elevation is fairly consistent throughout the reach so when locating it on the stream bank, look up and down stream for more indicators. Let’s run through a few examples of identifying bankfull in the field. The bankfull indicators highlighted in each photo are the main indicators that may be used to identify bankfull for each situation. Where is bankfull is located on this stream? What indicators can be used to identify bankfull? This is where bankfull is located, a change in slope and vegetation can be used to identify it. How about on this stream? This is where bankfull is located. A change in slope, a change in vegetation type, and the staining of rocks can be used to identify it. How about on this stream? This is where bankfull is located. A change in vegetation type and the elevation of undercut banks can be used to identify it. Where is bankfull on this stream? This is where bankfull is located. A change in slope and vegetation type can be used to identify it. This concludes the module on bankfull. The next critical concept this tutorial introduce is floodplain The floodplain is a relatively flat surface adjacent to a stream or river that is formed by periodic flooding. In many systems, this will be located at the same elevation as bankfull. Floodplains are important because they support riparian ecosystems, provide critical habitat for aquatic dependent species, and dissipate flow and energy during high flow events. Floodplain height is measured at all 11 main transects. Not all floodplains, however, are located at the same elevation as bankfull. In some systems, a stream has been cutting deeply into the stream banks. This down cutting is called incision and it can impact where floodplain is measured. Sometimes, in streams that are actively incising, stream waters never reach their floodplains, making these floodplains disconnected. Floodplain measurements in these systems can be a lot higher than bankfull height measurements. In some cases, streams will stop incising and begin to heal from the incision event and start to form a new, lower floodplain. The lower floodplain will be the one measured in Aquatic AIM sampling Unlike bankfull, floodplains are not always continuous throughout a reach, and the floodplain may alternate between left and right bank throughout a system. The first flat depositional feature at or above bankfull is measured, even if this is at a different elevation or side of the stream than the previous location. Floodplain height can also vary depending on what side of the stream it is on. For the Aquatic AIM protocol, the height of floodplains is always measured on the lower floodplain because the lower floodplain will be inundated first during a flood event. In some systems, especially those in mountainous regions, streams flow through V shaped valleys. These systems typically have no flat depositional surface above bankfull. In these systems, there is no floodplain; the height of the floodplain is recorded as the same height as bankfull. Can you spot where the floodplain is on this stream? How about bankfull? Which side of the stream would you measure floodplain? This is where the floodplain is located, the floodplain would be measured on the left side. Where is floodplain and bankfull on this stream? This is where the floodplain and bankfull is located, floodplain would be measured on the left side. Where is floodplain and bankfull on this stream? This is where bankfull and floodplain are located, floodplain would be measured on the right side. Scour line is the water height corresponding with average annual flow in a lotic system. Identifying scour line correctly is important because it is used to delineate the extent of several measurements that are taken throughout a reach. Scour may be located above, at, or below the current water level. When identifying scour line in the field, first try to evaluate it in a straight section of the stream. Scour is always below bankfull and is relatively consistent through the reach. Like the bankfull identification process, there are several indicators that are used to identify scour. Scour is often located on the ceiling of undercut banks and at the lower limit of sod forming vegetation. It can also be identified by indentations on point bars or by a vertical drop on the bank below bankfull. It is important to use a combination of these indicators because scour can be fairly hard to determine, even for a seasoned Aquatic AIM technician. This is especially true for those working in desert systems. Can you identify scour line in this system? How about bankfull and floodplain? These features are located here. Where is scour, bankfull, and floodplain for this stream? These features are located here. Where is scour, bankfull, and floodplain for this stream? These features are located here. This concludes the in-depth look at the critical concepts used in the Aquatic AIM protocol. If you are collecting Aquatic AIM data or would like to see more examples of bankfull, floodplain, and scour, please review the content by completing the practice quiz that accompanies this video.

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