Bob Leavy's historical research revealed some information on the construction of the "1910" dam, which was purchased and rebuilt by the Rust Pond Association in 1950. Its construction was required after the partial failure of the earlier dam.
The following article from the January 22, 1910 issue of the Granite State News (GSN) describes the incident:
"Saturday afternoon last our village was threatened for a spell with serious consequences, especially in the lower portions of the village, caused by the water from Rust Pond, some thirty feet above the village, finding an outlet under the dam, washing out some 12 feet, and rushing in torrents into the village. By the prompt action of Willie Tucker and a crew of men, dumping bales of waste, and working all night, drawing sand and excelsior, the damage was avoided Sunday afternoon. It is estimated that the cost will amount to $2,500 to check the flow of water. The cause is considered to be muskrats, working under the mud-sill of the dam."
The fact that the dam failure was due to muskrat burrowing, suggests that it was, at least in part, an earthen structure. The anticipated expenditure of $2,500 (in 1910) to control the flow of water, shows that substantial construction was planned. It appears that the owner, South Wolfeboro Woolen Mills, decided to build an industrial dam with an upper gate house and lower penstock, to supply water to their mills and other manufacturing businesses along Mink Brook. The new Bowers "History of Wolfeboro" cites various GSN articles indicating persistent low-water conditions in Rust Pond and other local water bodies around the turn of the century. I believe that these conditions, which required manufacturers to use more expensive steam or electric power to run their factories, led the Woolen Mills to increase the capacity of Rust Pond by constructing a higher dam. This is the structure which the RPA purchased in 1950.
Immediately after purchasing the dam, the Association paid Fred E. Varney $2000 to reconstruct the 1910 dam in accordance with plans developed by NH Water Resources Board (now NHDES Water Division). The reconstruction effectively changed the structure from an industrial "mill dam" focused on pond water capacity to a flashboard- controlled "recreational dam" focused on pond water level. Mill dams are dynamic in that the water level is quite variable depending on the amount of water used by the factories. Recreational dams, on the other hand, are more static with the water level normally held in a narrow range by periodic adjustment of the number of flashboards or "stop logs."