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These obstructions can impede floodflow, causing conditions that can raise flood heights upstream of the obstruction and increase velocities downstream of the obstruction. Brush, barrels, propane tanks, cribbing planks, and other debris can be carried downstream to collect at bridges or other structures, creating a damming effect and overbank flow. As floodflows increase, masses of debris may break loose and cause a wall of water and debris to surge downstream until another obstruction is encountered. In some instances, debris may collect to the point where structural capacity is exceeded and a bridge is destroyed or abutments and approaches are eroded and roadbeds damaged. In the study reaches, a particular problem exists with plugging (caused by mobile streambed material, wood, and other debris) of the grates that serve as inlets to the subsurface conduits on Chase and Gregory Gulches. In addition, because of the low base flow in each waterway, people have built structures, especially bridges, that encroach on the channel itself, thereby further restricting this floodflow capacity. Although there is some variation among sources in what constitutes a flood, documented floods have occurred in 1864, 1881, 1883, 1898, 1910, 1942, and 1975. High water also apparently occurred in 1887, 1914, 1936, and 1978, which might be considered as floods. Peak flood discharges and stages are unknown. It is important to note that apparently there has been no overbank flow along North Clear Creek in the study reach in approximately the last 70 years. However, frequent street flooding occurs in Chase and Gregory Gulches, largely due to the obstructions previously mentioned. Table 1 lists the flooding sources and dates of historic floods. It should be noted that the recorded flood on May 7, 1898, was not due to a thunderstorm but to a "flood from water from the reservoir which supplys the mills which was being sluiced out." The water "flooded the flume and two Black Hawk workers drowned in the unexpected torrent." Information regarding past floods was taken from newspaper accounts and interviews with several longtime residents of Black Hawk. In general, the typical effects of flooding included washing out of streets, sidewalks, culverts, and flumes, as well as substantial mud and debris deposition, often in building cellars. Less fre- quently, portions of buildings and their contents were destroyed or railroad tracks torn up. Several deaths have been caused by floods over the years. On May 29, 1864, the Central City Miner's Register account of the flood at that time notes that the Idaho mill was temporarily closed due to damages, that the road down North Clear Creek was destroyed, that every bridge was gone, and all communication between Black Hawk and Denver was cut off. The deleterious effects to the community are vividly described. 6