If horses drink from streams, make sure the water is still flowing. We don't recommend ponds or still water as water sources. But if you use a pond, check it. If it's drying up, the receding water leaves mud, which can be hazardous to horses, especial ly young foals. Also give horses a place to retreat from the direct sun on these 90-degree days.
Large trees can offer shade.
Or, a run-in or open shed is a simple way of providing shelter. Another advantage of a run-in shed is that some flies that bother horses in s ummer don't like the dark. So they stay out of sheds. If you keep stalls open, make sure the doors are wide enough to prevent injuries. Some pastures may become dry and provide limited forage.