When a stream’s discharge increases, both the water level (stage) and the velocity increase as well. Rapidly flowing streams become muddy and large volumes of sediment are transported both in suspension and along the stream bed. In extreme situations, the water level reaches the top of the stream’s banks (the bank-full stage, see Figure 13.3.4), and if it rises any more, it floods the surrounding terrain. In the case of mature or old-age streams, this could include a vast area of relatively flat ground known as a flood plain, which is the area that is typically covered with water during a major flood. Because fine river sediments are deposited on flood plains, they are ideally suited for agriculture, and thus are typically occupied by farms and residences, and in many cases, by towns or cities. Such infrastructure is highly vulnerable to damage from flooding, and the people that live and work there are at risk.

Most streams in Colorado have the greatest risk of flooding in the late spring and early summer when stream discharges rise in response to melting snow. In some cases, this is exacerbated by spring storms. In years when melting is especially fast and/or spring storms are particularly intense, flooding can be very severe. Alongside that, there are several other factors that can cause flooding events (other than the obvious torrential downpour):

1. High groundwater tables: Discussed more in depth in the following chapter, the groundwater table is the depth that groundwater is reached underground from the surface. If the groundwater table is high, it is very close to the surface (similar to if you are digging a hole and water starts seeping out, you have reached the water table). With additional precipitation soaking into the ground, it will not take much for the groundwater table to breach the surface and start a flood (similar to if you pour water in a cup that is already nearly full).
2. Shallow subsurface rich in clays or shales: due to their very fine particle sizes, water has a difficult time passing through in between clay particles. Thus, water cannot soak into the ground quickly, and will pond up towards the ground surface very readily.
3. Very slow moving weather fronts: even storms that dump light amounts of rain can contribute to floods. If the supply of precipitation over an area is slow and steady, with the supplying weather front being very slow moving, water soaking into the ground can start to accumulate towards the ground surface, even if the soil is efficient in absorbing precipitation.
4. Copious levels of urbanization or development: water on the ground has two destinations; soak into the ground or run off along the surface. With the development of more buildings, parking lots and paved roads, there is less ground of exposed soil that can soak up precipitation. Thus, surface runoff increases exponentially eventually draining into nearby streams. The excess amount of new water can easily exceed a streams bank-full stage.

One slight exception to the above seasonal trends of potential floods, and perhaps the worst flood event in Colorado history, took place along the Front Range from September 11, 2013 with lingering effects lasting into early 2014. The event started with a slow-moving (nearly stationary) cold front parked over the state, in which a humid tropical front, moving in from the south, collided with it. The result was nearly a week of literal non-stop precipitation. Nearly the entire Front Range was affected in some magnitude ranging from the Colorado-Wyoming border all the way down to Colorado Springs. The hardest hit area ended up being Boulder County with nearly 17 inches of rain recorded in four days (the area’s annual average precipitation is around 20 inches). 14 counties declared a state of emergency. When the floodwaters receded, over 1,500 homes were destroyed with 17 times more homes suffering damage to some degree. 19,000 people in total had to be evacuated and over 200 miles of roads were damaged or destroyed (Figure 15.5.3.).

Another hard-hit area was the Poudre River, running through Fort Collins. Although the city only measure just above five total inches of rain, the flooding occurred in all directions leading out from the town. In other words, flooding was not localized to one area of the city. The floodwaters made it such that Fort Collins was physically cut off from all other areas around it. Even access from Interstate 25 was impossible as the freeway crosses over the Poudre River from the neighboring town of Loveland. As a result, one improvement project that was undertaken was to raise the Poudre River/I-25 bridge eight feet, which became one element of the I-25 express lanes improvement project.

Figure 13.5.2 Variations in discharge of Boulder Creek for 2013, with severe flooding starting in early September. Note the change in the magnitude of the discharge axis between this graph and that shown in Figure 13.5.1. The highest discharge recorded was around 9,000 ft³/s, nine times more than the highest discharge for 2021.

Figure 13.5.3 A section of damaged road in Jamestown, CO after the 2013 Front Range floods.

One of the things that the September 2013 Front Range floods teaches us is that we can’t predict when a flood will occur or how big it will be, so in order to minimize damage and casualties we need to be prepared. Some of the ways of doing that are as follows:

• Mapping flood plains and not building within them
• Building dykes or dams where necessary
• Monitoring the winter snowpack, the weather, and stream discharges
• Creating emergency plans
• Educating the public

Media Attributions

• Figures 13.5.1, 13.5.2., 13.5.4.: United States Geological Survey, Public Domain.
• Figure 13.5.3: Wikimedia Commons