Tolay Lake was initially altered about 1850 by European settlers who dynamited a natural earth dam on the lake's south side, in order to enable more intensive agricultural cultivation and grazing of the basin; at times thereafter, the lake typically became almost dry in the summer months, when rainfall in this region is at its seasonal nadir. Another modern alteration is the construction of an east-west earthen causeway that effectively bisects the lake, impeding thorough water circulation and fragmenting plant and animal colonies of the lake.