Highlights What are the main findings? First seasonally explicit, multi-decadal (2000-2024) mapping of Ile River Delta (IRD) wetlands Multi-index ensemble with dynamic thresholds improves water classification accuracy Periodic and ephemeral surface water extent (SWE) dominates; stable SWE covers only 12% of IRD Significant summer-fall declines driven by reservoir regulation and warming What are the implications of the main findings? Findings support transboundary water cooperation that explicitly includes environmental flow allocations to protect downstream wetland extent, connectivity, and biodiversity. Evidence indicates reservoir and irrigation management should be coordinated basin-wide to better mimic natural seasonal flow regimes and reduce summer to fall wetland contraction. Results highlight urgent need for Conservation and Transboundary Cooperation in Arid Central Asia (ACA)Highlights What are the main findings? First seasonally explicit, multi-decadal (2000-2024) mapping of Ile River Delta (IRD) wetlands Multi-index ensemble with dynamic thresholds improves water classification accuracy Periodic and ephemeral surface water extent (SWE) dominates; stable SWE covers only 12% of IRD Significant summer-fall declines driven by reservoir regulation and warming What are the implications of the main findings? Findings support transboundary water cooperation that explicitly includes environmental flow allocations to protect downstream wetland extent, connectivity, and biodiversity. Evidence indicates reservoir and irrigation management should be coordinated basin-wide to better mimic natural seasonal flow regimes and reduce summer to fall wetland contraction. Results highlight urgent need for Conservation and Transboundary Cooperation in Arid Central Asia (ACA)Abstract Wetlands in dryland regions are rapidly degrading under the combined effects of climate change and human regulation, yet long-term, seasonally resolved assessments of surface water extent (SWE) and its dynamics remain scarce. Here, we map and analyze seasonal surface water extent (SWE) over the period 2000-2024 in the Ile River Delta (IRD), south-eastern Kazakhstan, using Landsat TM/ETM+/OLI data within the Google Earth Engine (GEE) framework. We integrate multiple indices using the modified Normalized Difference Water Index (mNDWI), Automated Water Extraction Index (AWEI) variants, Water Index 2015 (WI2015), and Multi-Band Water Index (MBWI) with dynamic Otsu thresholding. The resulting index-wise binary water maps are merged via ensemble agreement (intersection, majority, union) to delineate three SWE regimes: stable (persists most of the time), periodic (appears regularly but not in every season), and ephemeral (appears only occasionally). Validation against Sentinel-2 imagery showed high accuracy F1-Score/Overall accuracy (F1/OA approximate to 0.85/85%), confirming our workflow to be robust. Hydroclimatic drivers were evaluated through modified Mann-Kendall (MMK) and Spearman's (r) correlations between SWE, discharge (D), water level (WL), precipitation (P), and air temperature (AT), while a hybrid ensemble-occurrence framework was applied to identify degradation and transition patterns. Trend analysis revealed significant long-term declines, most pronounced during summer and fall. Discharge is predominantly controlled by stable spring SWE, while discharge and temperature jointly influence periodic SWE in summer-fall, with warming reducing the delta surface water. Ephemeral SWE responds episodically to flow pulses, whereas precipitation played a limited role in this semi-arid region. Spatially, area(s) of interest (AOI)-II/III (the main distributary system) support the most extensive yet dynamic wetlands. In contrast, AOI-I and AOI-IV host smaller, more constrained wetland mosaics. AOI-I shows persistence under steady low flows, while AOI-IV reflects a stressed system with sporadic high-water levels. Overall, the results highlight the dominant influence of flow regulation and distributary allocation on IRD hydrology and the need for ecologically timed releases, targeted restoration, and transboundary cooperation to sustain delta resilience.