Why Rivers Flood


Guest blog post by Katie Jagt

Katie Jagt is a consulting engineer to American Rivers and in 2010 was awarded a Fulbright Fellowship to study flood management in The Netherlands.


For millennia, floods have intrigued the human mind. No fewer than five civilizations have incorporated great deluges into their creation stories and it is not hard to understand why the power of water has shaped human culture when one stands on the rim of the Snake River Canyon in south-eastern Idaho and reflects on the fact that the sweeping and spectacular landscape was carved by a single, massive flood at the end of the last ice age.

Though ancient floods of this magnitude are hard to conceptualize, we are repeatedly exposed to tales of their contemporary brethren and the massive destruction they leave in their wake. Rivers and creeks flood when pulses of rainfall and/or snowmelt move downstream. This causes water to overtop the channel’s banks and spill onto the neighboring floodplain.

A natural river channel is shaped by the amount of water and sediment that travels through it. Even though rivers can vary greatly in their form, a natural river’s channel is almost always sized to carry the largest amount of water that flows through the system once every two years.

Put another way: rivers in their natural state are “designed” to flood in approximately HALF of all years. While this regular wetness created major problems for settlers attempting to farm in the river’s floodplain or establish homes and businesses nearby, the species that call floodplains home depend on this frequent inundation for their survival.

Major levee systems were built along our rivers starting in the mid-nineteenth century. This had the advantage of drying out floodplains which allowed farms, homes, schools, and businesses to move in as the chance of unwanted water on this land was reduced to 1 or 2 percent per year. But because of the massive investment in these areas behind the levees, the damage when a flood did occur became catastrophic both in terms of economic cost and human causality.

In addition, because levees isolate rivers from their floodplains, there are severe negative impacts to the species that spent thousands of years evolving to thrive in these dynamic floodplain environments. Smart solutions to our flood problems are multi-faceted. To truly manage the risk of flooding we must consider solutions that integrate structural controls like flood bypasses, levee setbacks and floodplain reconnections with management plans that keep large investments and human lives out of the most vulnerable areas. Furthermore, public education and outreach regarding risk is essential for safe communities.

It is only through incorporating these many pieces that we can sustain truly secure communities and make legitimate efforts to restore and rehabilitate our riparian ecosystems.