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3. EROSION AND SEDIMENTATION MONITORING RESULTS A site by site summary of erosion and monitoring results is presented. The results are broken into four categories: River cross sections, constructed inlet/outlet monitoring, bottomland monitoring cross sections, and qualitative observations of the hydraulics and erosion/sedimentation. Bonanza Bridge -Three years of river cross section data have displayed some changes to the channel morphology near the Bonanza Bridge bottomland. The most obvious would be the development of a large sand bar next to the Bonanza Bridge inlet. Approximately 2' to 3' of aggradation occurred over the sand bar. However, the development and shifting of large bars or macroforms is very common to this reach of the river. The 1997 runoff season saw the aggradation of the sand bar, however, 1998 saw 0.5' of scour of the bar along the bank and the shifting of the bar further out into the channel. The results of the all cross section surveys are presented in tables 1a-1d. Two sets of cross section surveys and two monitoring topographic surveys were taken at the Bonanza Bridge opening in 1997 and 1998. Figure 1 shows the Bonanza Bridge bottomland topography as built, with a flooding elevation of 4705 feet and flooding discharge of 13,000 cfs. Sedimentation processes in 1997 deposited up to 2.5' of sediment along the mouth of the opening. The magnitude of the deposition can be attributed to the long connection duration of the bottomland to the river. The maximum stage in 1997 was approximately 4709, resulting in a 4' depth across the inlet during the peak flow. This depth allowed the significant deposition experienced. The bottomland can fill completely within of couple of days. High flooding flows were experienced for approximately 50 days in 1997 and during this period, flows would circulate around the inlet dropping out sediment in the middle. However, two small channels developed along the upstream and downstream edges of the opening as flows receded. These channels helped to maintain the as-built floodability of the opening. This was a common phenomenon at many of the constructed bottomland inlets. Figures 2 and 3 show the location and magnitude of the deposition. During 1998, there was limited deposition at the mouth of the inlet. The maximum flood stage was 4706, allowing less than one foot of depth across the inlet. The small scour channels did become more developed, as flows receded. The resulting flooding discharge through these channels remains at 13,000 cfs. Cross section BB-5B, across the opening approximately 200' back from the top of bank (see appendix A) did indicate some minor deposition (up to 0.5') across the middle of the inlet/outlet. Some of this deposition may have been experienced in late 1997 after the final BB-56 cross section survey was pertormed. This cross section also shows the scour of the side channels. The 1997 and bottomland monitoring cross sections indicated little or no deposition of sediment in the bottomland. The river flows have to travel 1000' to 2500' from the mouth of the opening to reach the Bonanza Bridge bottomland. The monitoring surveys verify that the suspended sediments have mostly settled out in the reworked overbank area and do not reach the bottomland. Flooding observations were recorded for the Bonanza Bridge bottomland in 1997 and 1998. These observations include notes on the construction phase, as-built observations, and 1997 and 1998 flooding observations. The notes .are appended in the accompanying report, Volume IIB, Post-Restoration Sedimentation and Erosion Monitoring/Evaluation for Green River Floodplain Habitat Restoration Sites, near Vernal, Utah Technical Report (FLO Engineering, Inc., in print). Photos 1-4 show the Bonanza Bridge site as built, flooding at 22,000 cfs, and post-runoff 1997 and 1998, respectively. Observations in 1998 documented very little growth of the tamarisk carpet evident in the 1997 post runoff photos 3 and 4. Tamarisk seedlings have covered most of the opening, but it appears that the flooding and sand accretion in the inlet limits their growth. In fact, a line of cottonwood seedlings noted in 1997 showed more growth than the tamarisk seedlings deposited in 1997. Similar projects have indicated that tamarisk growth is limited by sand accretion while cottonwoods can continue to grow given these conditions. This may prove beneficial to the restored sites assuming these sites continue to flood annually or semi-annually per • design, keeping tamarisk growth in check. 3-1