Stream Impacts
Many commenters objected to the proposed NWPs, stating that thousands of feet of stream bed could be channelized or filled under these NWPs. These commenters said that linear foot limits for stream bed impacts should be imposed on the NWPs instead of acreage limits. A large number of commenters recommended adding a 250 linear foot limit for stream bed impacts to the new and modified NWPs. Other commenters suggested linear stream bed impact limits of 200, 100, and 50 linear feet. A few commenters said that the NWPs should not authorize any stream impacts. Another commenter requested clarification regarding the PCN thresholds for linear feet of stream bed impacts, asking if the flooded area is included with the filled area.
After consideration of these comments, we have decided to impose on NWPs 39, 40, 42, and 43, a 300 linear foot limit for filling or excavation activities in stream beds. This 300 linear foot limit applies only to stream beds that normally have flowing water. Division engineers can regionally condition the NWPs to lower the 300 linear foot limit for stream bed impacts, impose linear foot limits for stream bed impacts on other NWPs, or establish lower PCN thresholds for filling or excavating stream beds.
Several commenters stated that all Corps districts must use the same method to determine where the average annual flow of a stream is 1 cfs. One of these commenters recommended using drainage area as a substitute. Another commenter suggested that the guidance in the preamble to the final rule for the NWP regulations (33 CFR part 330) published in the November 22, 1991, Federal Register (56 FR 59112) should be used to establish where the 1 cfs point of a stream is located. That guidance described how to determine the geographic location of the limit of headwaters for perennial, intermittent, and ephemeral streams.
District engineers will utilize the best methods available to identify where the average annual flow of a stream is 1 cfs. Although the guidance published in the November 22, 1991, Federal Register was intended to assist district engineers and the regulated public in identifying the geographic location of headwaters (i.e., where the average annual flow is less than 5 cfs), this guidance can also be used to locate the 1 cfs point on a stream. District engineers can utilize the median flow, rather than the average flow, to establish where the 1 cfs point on a stream is located. This approach recognizes that streams with highly irregular flows, such as those occurring in the western portion of the United States, could be dry at the 1 cfs point for most of the year and still average, on an annual basis, a flow of 1 cfs because of high volume, flash flood type flows which greatly distort the average. Furthermore, we recognize that using the median flow for an entire year in streams that have no stream flow for over half the year but with flows greater than 1 cfs for several months would also distort the average. It should also be noted that precision is not required in establishing the 1 cfs point. The definition allows the district engineer to use approximate means to compute it. The drainage area that will contribute an average annual flow of 1 cfs can be estimated by approximating the proportion of average annual precipitation that is expected to find its way into the stream. Knowing the amount of area that will produce this flow in a particular region, the 1 cfs point can be approximated from drainage area maps. For example, in most areas of the eastern United States (i.e., east of the Mississippi River), one square mile of drainage area produces 1 cfs of stream flow annually.