EMERGING GEOLOGICAL PERSPECTIVE ON THE REPUBLICAN RIVER AND ITS GREAT FLOOD OF 1935: PRE-DAM AND POST-DAM CHARACTERISTICS

Effects of the1935 Republican River flood—the last pre-dam, valley-wide flood—were captured in aerial photographs as early as 1937. The river’s upper reach (Benkelman-McCook, Nebraska) was then a low-sinuosity (S = 1.09) single-channel stream, bifurcating into an anabranching planform only in a 5.5 km stretch east of Stratton. Point bars (many downstream-migrating) and local clusters of in-channel bars were common. The flood appears to have occupied and partially filled multiple secondary channels in the upper reach, perhaps briefly conferring a braided planform. Approximately 20% of the available area of the floodplain and channel belt in the upper reach was covered by new sediment (sand) after the flood. Downstream (McCook to 6 km west of Holbrook, Nebraska) the river exhibited alternating, 6-15 km-long, low-sinuosity and meandering stretches, the former showing flood effects similar to those described above. Yet farther downstream to Junction City, Kansas, the river was dominated by a meandering planform. It is likely that ~25 meander cutoffs formed during the 1935 flood in the 175 km between Holbrook and Superior, Nebraska. At least 37 channel splays, many clearly transitioning downgradient into anabranching flood channels—forming complexes > 1 km in length—also formed in this stretch and evidence for recent point-bar accretion is widespread in the earliest aerial photographs. A major avulsion during the flood produced a new 28 km-long channel of lower sinuosity (S = 1.20) than the equivalent pre-flood channel (S = 1.75) between Edison and Orleans, Nebraska. We speculate that valley-wide floods with approximately centennial recurrence intervals exerted a very strong influence on the geomorphic and sedimentologic characteristics of the Republican River. Post-dam changes on the river are also striking and include widespread channel narrowing and, within much larger pre-dam meanders, the development of meanders an order of magnitude smaller.