A flood is an overload of the capacity of a drainage area.

The series of converging streams that comprise a drainage area are fractals that typically have a dendritic pattern resembling the structure of a tree. This shape is the most efficient way to collect or distribute material over a surface area or throughout a volume, and so it occurs as a basic shape in many natural processes.

Streams within a drainage area are classified according to how many branches removed they are from the ultimate destination of the water, which is the sea. The Mississippi is an order one stream because it discharges into sea level in the Gulf of Mexico. The Ohio and Missouri rivers are second order streams because they discharge into the Mississippi. One can trace third, fourth, fifth and higher order streams upslope on a map down to the smallest rill.

Flow gauges placed in higher order streams provide numeric data that can be crunched into predictions of water level rise and further refined as the flood crest moves downstream.

As the floodwaters rise, measurements of the flow rate continue to add data to the overall dynamic of the flood condition. These numbers help to predict the potential flood stage levels downstream and allow residents there to take appropriate measures.

Using combined data from many different gauges and flow meters, hydrologists track the movement of the flood crest downstream.

When the amount of water in the drainage area exceeds the capacity of the stream channels and the stream overflows its banks, the word “;flood”; takes on a second and more drastic meaning.

Most stream channels meander within a broad flood plain, which is a flat area many times wider than the stream itself. Flood plains are where streams store fine-grained silt and clay. A stream forms its own flood plain over thousands of years by widening and depositing sediment in its valley when it has overflowed its banks.

Many streams have built a natural levee that allows the level of water in the stream to be higher than the surrounding flood plain. Once the water breaches this levee, the flood plain floods, depositing fine-gained silt and clay.

This becomes a problem only when it affects human life. People like to live on the flood plains where the land is flat and the soil is fertile. This is beneficial except when the flood plain actually floods.

We tend to react as if the flood plain is off-limits, reserved for humans only. Living on a flood plain, people seem to forget that it is a part of the stream. We try to prevent the stream from using it by building confining channels out of concrete.

As much as we try to hold back the waters, and as much as one feels the tragedy that loss of life and property due to flooding brings, we are always humbled by the reality that somewhere there is always a flood that is bigger than any before it.

Floods are yet another reminder that nature always bats last.

Richard Brill is a professor of science at Honolulu Community College. E-mail questions and comments to .(JavaScript must be enabled to view this email address).