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protection from erosion. They also play an important part in habitat complexity today in bank shading, root-wads and debris piles. Construction and operation of Navajo Dam has had an impact on the flood frequency of the river, especially in the portion above Four Corners. Pre- and post-dam flood frequency analyses indicate that the magnitude and frequency of flooding at Shiprock has been greatly reduced. For example, the 100 year flood was reduced by 755. The effect at the Bluff gage was much different. Since the large magnitude floods are usually produced by storm events in late summer and fall, the lower part of the river is less affected by the dam. At this lower gage, the 100 year flood was reduced by only 35~. Reduction in peak flooding would contribute to the stability of the river channel. While considerably more channelized than in the early part of the century, the river is .still dynamic. Analysis of aerial photography from 1988 to 1992 indicates continuing adjustment in channel alignment, although at a reduced rate compared to the situation in the first half of the century. Narrative history would lead us to believe the channel described around the turn of the century and as seen in the 1934 photography was not typical of the channel earlier in history. Unfortunately, we do not know the condition of the river in these times prior to the increased sedimentation documented in the literature. As a more natural hydrograph is returned by re-operation of Navajo Dam, there is some uncertainty as to the effect on channel dynamics. In March 1992, 11 river cross-sections were established to allow measurement of channel change in relation to different hydrographs. Surveys of these cross-sections in March and again in July demonstrated that scour exceeded deposition on average during the spring runoff period in 1992. However, the scour was not excessive for the 1992 hydrograph. Results from these cross- section surveys over the duration of the study will be compared to long term cross-section measurement at USGS gages for comparison to long term trends. Sediment sampling at 9 sites along the San Juan River at 5 times during the runoff period indicated a general increase in sediment concentration with distance downstream, with the largest increase between Shiprock and Four Corners. Sampling was done during non- storm periods with little or no tributary inflow below Shiprock to assess the capacity of the river to remove sediment from the system. The increase in suspended sediment with distance down river corresponds with the net scour from the cross-section measurements. While further sediment sampling is needed to quantify the capacity for sediment transport, early indications are that the system does have excess sediment transport capacity during the peak runoff period, especially in the reach from Navajo dam to 15