We present a series of laboratory experiments in which hot, dense aqueous solutions were cooled and crystallized by contact with cold ambient solutions as they flowed on horizontal, sloping, or nearly vertical boundaries. In each case, the crystals grew to form ''dams'' on the boundary, behind which the input fluid ponded before overflowing the rim. On a sloping boundary, simultaneous crystallization of a number of dams led to the growth of terraces that are reminiscent of natural travertine and sinter terraces. This phenomenon is not critically dependent on the composition, concentration, or flow rate of the fluid. If the fluid overflowing the rim becomes undersaturated, uniform crystallization is replaced by dissolution that soon focuses at one point of the rim, leading to a ''breakout'' of the ponded fluid. In an appendix, we also examine quantitatively the flow of water over the rim of mound springs and terraces and show that surface tension can determine both the two-dimensional volume flow rate and the length of rim overflowed. ¿ American Geophysical Union 1996