A dry creek bed, also known as an arroyo, wash, or wadi, refers to a streambed that only contains water when it rains. When it is not raining, a dry creek bed appears as a linear depression in the landscape with a sandy, rocky bottom. Dry creek beds are common in arid and semi-arid environments where rivers and streams are not constantly flowing. Let’s take a closer look at what defines a dry creek bed and what they typically look like.
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Definition of a Dry Creek Bed
A dry creek bed is a type of intermittent or ephemeral stream that only flows with water after precipitation events like rainstorms or rapid snowmelt. When there is no rain or snowmelt occurring, these streams are dry. Dry creek beds are characterized by:
- A linear depression in the landscape that looks like an empty streambed
- A sandy or rocky bottom with no water present
- Banks or channel walls along the edges of the streambed
- An upstream drainage network that feeds into the channel during flows
- An intermittent flow regime, where water is only present for days or weeks at a time after precipitation events
- Being located in arid to semi-arid environments that cannot support perennial streams
When it rains over a drainage basin, water accumulates in the dry streambed, causing flash flooding for a brief period before drying up again. These transitory flows are capable of eroding and transporting sediment along the channel. Between flows, the dry creek bed sits empty.
Typical Features
While dry creek beds can look different depending on the local environment, there are some common features found in most dry wash systems:
- Streambed shape: The streambed is usually wide and shallow, with a flat and sandy bottom. The cross-sectional shape resembles a wide, upside-down teardrop.
- Channel walls: Taller banks or short channel walls line each side of the streambed. These channel walls confine ephemeral flows.
- Sediment deposits: Deposits of loose, coarse sediment like sand, gravel, cobbles, and boulders litter the streambed. These sediments were transported and deposited by previous flows.
- Debris: Debris piles consisting of fallen trees, branches, and bushes accumulate along the banks and bottoms of dry wash channels.
- Vegetation: Hardy desert plants, shrubs, and grasses grow along the edges and banks of dry creek beds when moisture is available.
- Rocky materials: The streambed often contains rocky materials like boulders, cobbles, gravel, and bedrock outcrops.
- Drainage patterns: Braided patterns of small channels created by previous flows are etched into the sandy streambed.
- Animal tracks: Footprints of animals like coyotes, rabbits, and rodents that traverse the dry streambed may be visible.
Typical Profiles
Here are some of the most common profiles and cross-sectional shapes that dry creek beds exhibit:
Wide and Shallow
Most dry creek beds have wide, shallow streambeds that can be anywhere from 10 feet across to over 100 feet across. The width is much greater than the depth. This allows a large volume of water to pass through during flash floods. Streambed gradients are low, often less than 3%.
Narrow and Deep
In some cases, dry wash channels can be narrow and deep, sometimes taking the form of a gulley. These deeper arroyo channels form when flash floods erode the streambed faster than sediment is deposited. Streambed gradients are steep.
Braided Channels
Some dry creek beds contain multiple, small branching and intersecting channels across the streambed, creating a braided pattern. These braided channels form as water erodes sediments and creates new flow paths during flooding.
Meandering Channel
In rare cases where gradient is very low, dry wash channels can meander back and forth across a valley, forming sweeping curves and oxbow bends. This requires very low gradients, cohesive streambanks, and highly erodible soils.
Common Locations
Dry creek beds are most commonly found in certain environments and geologic settings, including:
- Arid regions like deserts and dry steppes
- Semi-arid environments like shrublands and grasslands
- Areas dominated by ephemeral stream networks
- At the bottom of arroyos or canyons
- Along the margins of intermittent rivers and lakes
- In valleys between mountain ranges and hills
- In regions with highly variable precipitation patterns
Dry creek beds are most extensive in the desert regions of the Southwestern United States and Northern Mexico. However, they can occur anywhere with an arid to semi-arid climate where waterflows are seasonal and inconsistent.
Appearance During Dry Periods
During the long periods between flows, dry creek beds have a very distinct dusty and desiccated appearance:
- The sandy streambed is completely dry with no moisture.
- Sand, gravel, and rocks make up the creek bed material.
- Dried and cracked mud or salt deposits may be present.
- Desert varnish stains the rocky surfaces.
- The channel is often overgrown with bushes, small trees, or grasses.
- Leaf litter and dead brush accumulates in piles.
- Tracks from animals cross the streambed.
- Dust, sand, and fine sediment coat the banks and bottoms.
These dry washes seem to be carved into the landscape, creating linear depressions clearly delineated from the surrounding terrain. The dry creek bed is defined by its banks rising above the sandy arroyo bottom.
Appearance After Rains
During and immediately after rainfall or rapid snowmelt, dry creek beds transform as they fill with rushing water:
- Water quickly flows down the drainage basin into the channel.
- The creek bed fills with turbulent muddy water laden with sediment.
- Water flows swiftly across the full width of the streambed.
- Debris piles wash downstream.
- Channel banks confine and direct the rapidly moving flows.
- Sediment is scoured from banks and the streambed.
- New sediment is deposited as the floodwaters fan out.
- High energy flash floods are capable of deep erosion.
- The flow eventually tapers off as rainfall ceases.
- Water slowly drains from the channel or evaporates as the flood ends.
These flash floods may only last for a few hours or a few days before the dry wash returns to its previous dusty state. The ephemeral waterflows leave behind a signature of erosion and deposition.
Geologic Origins
Dry creek beds form in certain geologic environments where ephemeral flooding dominates over perennial flows. The key factors leading to their formation include:
- Arid to semi-arid climates with low annual precipitation.
- Highly variable rainfall patterns over seasons or years.
- Landscapes where drainage basins exceed water supplies.
- Steep drainage gradients that concentrate ephemeral floods.
- Soils and sediments that can be eroded into channels.
- Sparse vegetation and lack of permanent waterflows.
- Flat valley bottoms and plains that collect sediment.
Over geologic time, repeated flash flooding events carve out dry wash channels by erosion. Seasonal or annual floods transport loads of sediment into streambeds, forming the characteristic sandy, gravelly deposits.
Role in Arid Landscapes
Dry creek beds play important roles in the arid environments where they are found:
- They efficiently deliver rainwater through drainage basins and valleys.
- Flash floods redistribute sediments eroded from banks and surrounding hillslopes.
- Sediments deposited in washes increase soil fertility and moisture retention when rains permeate into them.
- Groundwater can be recharged as floodwaters infiltrate into sandy streambeds.
- In some cases, washes store floodwaters for months at a time.
- Dry wash environments provide specialized habitats for desert plants and animals.
Although dry most of the time, ephemeral channels are still an important part of arid landscapes. They shape terrain, foster life, and control water flows during brief wet periods.
Unique Hazards
Dry creek beds pose special hazards related to their rapid and unpredictable flooding:
- Flash floods can occur suddenly, even when no storms are visible upstream.
- Floodwaters rush down channels at dangerously high speeds.
- Dry washes can easily overtop their banks during floods.
- The rocky, debris-filled currents can wash away vehicles and infrastructure.
- People can be caught off guard and swept away by quickly rising waters.
- Tunnels or culverts designed to divert flows can get clogged and fail.
- Banks often collapse as floodwaters scour and undermine soils.
Care should be taken around dry streambeds to avoid the hazards of flash flooding, especially during the rainy season or extreme weather events.
Conclusion
In summary, dry creek beds are sandy, rocky channels that temporarily fill with raging floodwaters after it rains. When no precipitation is occurring, they appear as linear trenches carved into arid landscapes, accumulating desert dust and vegetation. The unique hydrology of dry washes shapes the terrain and controls water resources in areas where rain falls sporadically. Recognizing ephemeral stream patterns is key to understanding the environments they occupy.