The San Juan River Ant Fall: What It Is and When It Happens

As of July 13, we officially have ants on the San Juan River.

Every year, anglers ask us the same question:

When is the ant fall?

The honest answer is that we do not know.

The ant fall is one of the most exciting dry-fly opportunities on the San Juan, but it is also one of the least predictable. Traditionally, it has been a once-a-year event. In recent years, however, we have seen fewer massive ant falls and more scattered, lighter falls throughout the summer.

What Is an Ant Fall?

During warm, wet summer conditions, winged reproductive ants emerge from their colonies. Some of them end up landing on the river, where the trout absolutely lose their minds.

The San Juan does not experience many truly large insect hatches, which is one reason the ant fall is so special. When enough ants hit the water at once, trout begin looking up and feeding aggressively on the surface.

Watching San Juan trout eat large dry flies with confidence is an incredible experience—especially on a river where anglers are often fishing tiny midges and light tippet.

When Does the Ant Fall Happen?

We have seen ants as early as May and as late as August 4. Most ant activity, however, occurs during the summer monsoon season, particularly in June and July.

The exact timing cannot be predicted, but there are a few conditions we look for:

Heavy rain creates puddles and saturates the ground. That rain is then followed by a hot, sunny day. When the ground begins steaming and moisture is visibly evaporating, the conditions may be right for winged ants to emerge.

That does not guarantee a major ant fall, but when those ingredients come together, it is worth paying attention.

What Flies Should You Fish?

When ants are on the water, it is time to put away the tiny dries and throw some foam.

Some of our favorite patterns include:

• Fat Alberts
• Micro Chubbies
• Stubby Chubbies
• Small black foam terrestrials
• Any dark pattern that roughly matches the size of the naturals

The ants can vary considerably in size—from roughly the tip of your pinky to nearly the length of your entire pinky. Carrying several sizes of black foam flies will help you match what the fish are seeing.

The ant fall is unpredictable, brief and sometimes frustrating—but that is also what makes it so exciting.

If you are fishing the San Juan during the summer, it rains enough to leave puddles overnight, and the following day is supposed to be hot, keep the dry-fly rod ready.

You might just get lucky.