In this research, 18 experiments were conducted to investigate the effects of sudden constriction and expansion on frontal hydrodynamics. The flow rate was set at 1.1 l/s, with variables including concentrations of 5 and 10 gr/l, slopes of zero, 1.5, and 3 percent, and width ratios of 0.39, 0.64, and 0.92. The flow regime encompassed all three states: subcritical, critical, and supercritical. In constrictions with ratios of 0.39 and 0.64, the front velocity initially increases up to a point due to buoyancy flux but then decreases under the influence of mixing. Increasing the constriction intensity and slope raises the maximum velocity and delays its occurrence. Specifically, at a 3 percent slope, the maximum velocity for the 0.39 ratio compared to 0.64 increased by 20.29 percent and 24.49 percent for concentrations of 5 and 10 gr/l, respectively. In expansions with these two ratios, the velocity decreases with distance due to intensified turbulent instabilities, with a more pronounced effect at the 0.39 ratio. At the 0.92 ratio, constriction and expansion had no significant impact, and the velocity trend resembled that of a constant-width channel: decreasing at zero slope and increasing in sloped channels. The front height increased with distance in both constriction and expansion regions for all width ratios, with greater intensity at smaller ratios. The effect of constriction is in support of flow driving force, strengthened by slope but weakened by concentration. In expansion, mixing-induced resistance prevailed, strengthened by concentration but weakened by slope.