The historical course of the life of the country is here compared with a river. Just as a river has a main channel where most of the water flows, and which is therefore deep and wide, so the course of the life of the country has a main channel, which is identified here as the lives of most of the population, ordinary people living in small communities. By contrast, a river has smaller side channels, where comparatively little water flows and which are therefore shallower. These are compared with the events of the court and the cities. This is exactly the opposite of the conventional view of history.

The water is metaphorically the progression of life and history. This is a very old metaphor, most famously used by the Greek philosopher Heraclitus. Here the idea is that the cities and the court, which are usually regarded as the places where the most important and influential part of history occurs, are actually less important than the communities where most people live, unaware of the court. So the small communities the author is referring to are like the bed of the river through which most of the water (life and history) flows.

The "deep, main current" is echoed later in the passage by the "main stream" (and the word "mainstream" in English means that which is regarded as normal and ordinary). The author plays with the concept of depth: the conventional view of history which concentrates on political events is shallow, superficial (staying on the surface of history), whereas the lives of ordinary people are deep (fundamental, important, voluminous and unknown).

This reversal of the ordinary idea of life in the cities as the "main stream" and small communities as "backwaters" (minor channels of the river of history) is the real point of the passage.