Surveys taken in the reservoir at Lake Oahe (190+ km) have surveys over a shorter time frame (1968–1989). Despite the shorter time frame the trends in reservoir channel change are still considered
applicable. The rate of change in the thalweg bed elevation was calculated as a function of downstream distance and year by determining the minimum elevation of each cross-section (or the maximum depth of the channel), subtracting it from the minimum elevation of the cross-section for the next available year of data, then Osimertinib dividing by the time interval between the two measurements (Eq. (3)). equation(3) BE t1−BE t2t1−t2where BE is the minimum bed elevation (m) and t is time (years). Channels vary naturally through space and time. To attribute a geomorphic change to an anthropogenic disturbance, it must be outside the range of the natural variability and should be statistically significant. This was calculated using the Williams and Wolman (1984) method;
ergodically assuming that longitudinal variation in a single year can approximate ZD1839 cost at-a-station variability through time. The mean pre-dam channel cross-sectional area along the entire segment (irrespective of the defined geomorphic zones) and standard deviation was calculated. The study included all cross sectional data available from 1946, which is the only year of the survey data before the dam was completed. The spatial standard deviation was used to approximate natural variability and compared to the changes at each cross sections. Historical photos from 1950 and 1999 were used to compare change in island area. Photos were georectified using ArcGIS version 10.1. The channel banks and islands were delineated for each year
and the aerial difference between the channel and island boundaries were determined. Water levels along the river vary due to seasonal and annual weather patterns, dam operations, Alectinib concentration tributary influx, and reservoir levels. This consideration is particularly germane with respect to sand bars as the area exposed (and therefore quantified) depends largely on flow depth. The 1999 photo set provides the best comparison to the pre-dam photos (1950) due to similar discharge rates from the Garrison Dam (841 and 835 m3/s respectively or ∼0.7%) and stage gage at Bismarck, ND. All other historical imagery available was collected with discharge differences of 10% or greater related to the pre-dam 1950 images. The spatial extent of the aerial photo analysis ranged from the Garrison Dam to the upper section of Lake Oahe (approximately 130 km downstream of the Garrison Dam); this is the farthest downstream extent of the 1950 images. Image quality of historical aerial photography is often poor, and distortion and clarity are common issues. The aerial photos from 1999 provided by USACE were orthorectified. These orthorectified images were used as a baseline to georectify the 1950 photo set. A minimum of 10 control points per 5 km of river were used.