River Birch Diseases
The clear liquid is probably "honeydew", which is excreted by a kind of aphid known as the spiny witch hazel gall aphid. This insect is extremely common on birches. It attaches to leaf undersides and sucks juices from the leaves.
Some years their populations are higher than others. It has many natural enemies (predatory insects) that keep its population down, so you don't want to spray a strong pesticide and kill the predator insects. That makes the aphid populations explode.
Honeydew is water soluable, so you can wash it away with water. Rain also washes it off.
If it's a big problem this year for you, you can spray with a horticultural oil. Be very careful to read the label, as spraying whem temperatures are too high can kill leaves. Spray under the leaves.