Water scarcity in the Tigris–Euphrates basin is increasingly accompanied by deteriorating irrigation-water salinity and strong spatial contrasts in land cover. This study evaluates river-salinity gradients together with Landsat-derived vegetation-cover patterns using four 2021 monitoring campaigns (March, April, June, and July), exact WGS 84 coordinates for 18 Tigris and 14 Euphrates locations, irrigation water quality index (IWQI) scores, and distance-based repeated-measures models. The updated coordinate–electrical conductivity (EC) workbooks contained 125 non-missing EC observations, all of which matched the quality-controlled analytical archive. Cumulative inter-station geodesic distance was 897.48 km for the Tigris monitoring sequence and 779.16 km for the Euphrates sequence. Mean EC was 1.08 ± 0.65 dS/m in the Tigris and 1.55 ± 1.06 dS/m in the Euphrates; mean IWQI was 87.78 ± 8.60 and 82.53 ± 15.47, respectively. Linear mixed-effects models with station random intercepts and campaign fixed effects estimated EC increases of 0.217 dS/m per 100 km in the Tigris (95% confidence interval: 0.174–0.259) and 0.385 dS/m per 100 km in the Euphrates (0.251–0.520), with corresponding IWQI declines of 2.79 and 5.60 units per 100 km (all p < 0.001). The Euphrates distance slope was significantly steeper for EC, total dissolved solids, and IWQI (river × distance p = 0.007, 0.004, and 0.002, respectively). First-order autoregressive correlation generalized estimating equations, leave-one-station-out analyses, omission of the three lower-Euphrates April gaps, exclusion of the disputed Al-Baghdadi IWQI value, and removal of Al-Madina independently preserved the downstream signal. Both marginal and conditional first-neighbor residual Moran tests showed no evidence of remaining longitudinal autocorrelation after false-discovery rate control. Landsat-derived normalized difference vegetation index mapped the basin-scale summer vegetation-cover pattern, whereas the optical salinity index was retained as environmental context rather than treated as a calibrated predictor of river chemistry. The results identify the lower Tigris and, more specifically, the Shinafiyah–Al-Madina sector of the Euphrates as priority reaches for salinity surveillance and adaptive irrigation-water management.