The Owyhee River in SE Oregon may be the world's showcase example of
lava deltas (well, certainly Oregon's). On numerous occasions in the
Quaternary, basalt flows from various sources have entered the river
canyon, formed dams, and created lakes. Stratigraphic evidence
suggests that some of these lakes lasted 10,000s of years before the
huge dams were finally breached...they have long downstream runouts on
the order of 10s of km and serve as very effective plugs (unless they
fail quickly...but that doesn't appear to be a tendency of lava dams
on the Owyhee).

The beauty of this process, other than the fact that it even happens
at all, is that the damming event is accompanied by a delta building
event. In fact, these two events are one in the same. The flow enters
the lake of its own creation; its front is chilled and pillows are
created along with a lot of glass fragments (hyaloclastite); further
advances of the lava auto-brecciate the flow front; the brecciated
zone of tumbled pillows avalanches into the depths of the lava dammed
lake, and a beautiful series of Gilbert-type foreset beds of tumbled
pillows and hyaloclastite material is formed.

There are many lava deltas along the Owyhee ranging in age from about
1.8 Ma to 60 Ka. The one in these images is the youngest. In the upper
image, the foresets dip upvalley and indicate the advance of the delta
upstream. Note the tributary channel (left) that is overrun by the advance of
the delta. The lower photo is to help assess the scale of the feature.

The elevation of the lava delta base above the modern river is the
amount of incision in the last 60ky or so.