Stream terraces are step-like landforms flanking river valleys, representing the remnants of former floodplains abandoned at a higher elevation above the current waterway. These benches consist of an upper, flat surface called a tread and a steeper slope known as a riser, which drops toward the next lower terrace or the active floodplain. Geologists study these features to reconstruct how rivers have responded to changes in climate, tectonics, and other large-scale environmental shifts. The height, composition, and age of a terrace sequence provide evidence of past river behavior and landscape evolution.
The Cycle of Erosion and Deposition
Stream terraces form when a river shifts from a period of sediment accumulation (aggradation) to one of intense downcutting (degradation). The process begins when the river deposits significant amounts of alluvium, building up a broad, flat floodplain. This depositional phase occurs when the river’s sediment load exceeds its ability to transport the material downstream.
Terrace formation is triggered when conditions change, causing the river to transition into vertical incision. During this downcutting, the river erodes its channel deeper into the deposited material or underlying bedrock. As the river establishes a new, lower floodplain, the older, higher surface is left stranded on the valley sides, forming the terrace. This alternation creates a sequence of terraces, often appearing as a staircase on the valley sides.
Major Environmental Drivers of Terrace Formation
Terrace formation is dictated by large-scale environmental changes that force a river to increase its erosive power. A widespread trigger is a change in regional climate, which alters both the river’s discharge and the sediment supply. For example, during periods of glacial melt or increased precipitation, higher water volumes give the river more energy to carve downward. Conversely, a shift to drier conditions can reduce protective vegetation cover, increasing sediment supply and causing the river to aggrade. This sets the stage for future incision when flow increases again.
Tectonic activity is a powerful driver, particularly in mountainous regions. Uplift of the land relative to the river channel increases the gradient, or slope. This steepening boosts the river’s velocity and erosive capacity, forcing it to cut downward rapidly to establish a new, lower equilibrium profile. Episodic uplift can result in a series of distinct terraces, with each one marking a pause in the tectonic movement before the next incision began.
A change in the river’s base level, the lowest point to which a river can erode, also initiates terrace formation. For rivers flowing into the ocean, a fall in global sea level forces the river to cut down at its mouth to meet the lowered base level. This wave of downcutting, known as headward erosion, progresses upstream, causing the river to incise along its length and abandon its former floodplain. This mechanism is significant for large river systems, where global sea-level fluctuations associated with glacial cycles have driven widespread terrace formation.
Types of Terraces and What They Reveal
Terraces are classified based on their composition, which provides clues about the erosional event.
Alluvial Terraces
Alluvial terraces, also called fill terraces, form when a river cuts down solely through its own previously deposited sediment. They are characterized by thick layers of river-deposited material, indicating a significant period of prior aggradation before incision.
Strath Terraces
A strath terrace forms when the river erodes laterally into the underlying bedrock, leaving only a thin veneer of alluvium on the rock surface. The presence of a strath terrace indicates the river possessed enough energy to cut through resistant rock, suggesting intense downcutting driven by uplift or high stream power.
The symmetry of the terraces across the valley also reveals the incision style.
Paired Terraces
Paired terraces are found at the same elevation on both sides of the river, indicating the river cut down rapidly and vertically with little lateral shifting. This morphology is associated with large, regional events like rapid climate shifts or uniform tectonic uplift.
Unpaired Terraces
Unpaired terraces occur when the terrace on one side of the valley does not match the elevation of the terrace on the opposite side. This asymmetry suggests the river was migrating laterally across the valley floor while simultaneously incising, often in response to localized erosion resistance or differential uplift.
Determining the Age of Terrace Sequences
To establish a chronological history of environmental change, scientists determine the absolute age of the terrace surfaces and the sediments they contain.
Absolute Dating Methods
One common approach is Optically Stimulated Luminescence (OSL) dating, which measures the time elapsed since quartz or feldspar grains in the sediment were last exposed to sunlight. OSL provides a direct age for the burial of the alluvium, constraining the timing of the aggradation phase that preceded incision. Radiocarbon dating provides ages up to approximately 50,000 years for organic materials preserved within the deposits, offering a precise timeline for recent river activity. To date the actual abandonment of the terrace surface, scientists employ cosmogenic nuclide dating. This method analyzes the buildup of rare isotopes, such as Beryllium-10, in minerals on the exposed surface, estimating how long the surface has been exposed to cosmic rays since the river cut away from it.
Relative Dating Methods
Relative dating techniques, such as assessing soil development and weathering intensity on the terrace tread, are also used to establish a sequence. Older terraces generally exhibit thicker, redder, and more complex soil profiles compared to younger ones, allowing for the correlation of terraces across a region. By combining these absolute and relative methods, geoscientists can create a detailed sequence, linking each terrace-forming event to a specific period of climate or tectonic forcing.