Garrett was one of the coauthors that has published on end member analysis, which was used in one study to estimate the percentage of the streamflow hydrograph that was rainfall, interflow and groundwater. If you don’t want to go to that trouble, and just want some informed estimate, you might try using conductivity. Rainfall usually has very low conductivity, groundwater moderate to high. Shallow interflow or low detention water table, streamflow or barrage backup water is likely In between. If the groundwater substrate also has low conductivity, then you might try the end member approach. How the barrages are managed is likely to affect the normal groundwater influence.

Reference - Garrett CG, Vulava VM, Callahan TJ, Jones ML. Groundwater-surface water interactions in a lowland watershed: source contribution to stream flow. Hydrol Process. 2012;26(21):3195–3206.