By very late dusk river SMB have moved into the shallows, increasing abundance considerably in comparison to the daytime population. The fish come to you. You only need to cast to water that is about 2.5 to 4 ft deep, but if you start earlier in the evening, casting farther out theoretically may intercept fish that are gradually heading toward shore. If flows are extremely low, as in drought or near drought conditions, this pattern may be less reliable because you may have large expanses of shallow, featureless flats in front of you.
There are river specific threshold values for flow that can really produce high numbers of SMB in the shallows when flows are equal to or higher than those thresholds. You’ll need to figure that out by not giving up the first time that you get skunked and paying attention to the gauges. These observations come from electrofishing surveys and “experiments” on Pa rivers to determine the best times to sample SMB that produce the highest catch rates. If water temp drops below about 12 deg C, forget about this pattern.