S.A. Schumm’s Episodic Erosion Model

Imagine a riverbed that remains stable for years and then suddenly experiences a major flood, causing rapid erosion and shifting sediment downstream. Unlike the classical idea that landscapes evolve gradually and continuously, S.A. Schumm proposed that denudation occurs in episodic bursts, leading to periods of rapid erosion followed by long phases of deposition. This irregular pattern creates complexity in landform development, rather than a predictable sequence of changes.

Key Principles of Schumm’s Model

1. Denudation is Episodic, Not Continuous
   • Landscape evolution happens in phases of instability (erosion) followed by stability (deposition).
   • A river valley, for example, may undergo rapid deepening during a flood (erosion phase), then stabilize as sediment deposits build up over time.
   • This cycle repeats, making the development of landforms complex and unpredictable.
2. Two Key Concepts Explain This Complexity:
   • Geomorphic Thresholds
     • Changes in landscapes are not always triggered by external forces like tectonic uplift; rather, they occur when internal thresholds are crossed.
     • Example:
       • Over time, a river deposits sediment in its channel.
       • As sediment builds up, the slope steepens beyond a critical limit (threshold).
       • When this threshold is reached, erosion suddenly accelerates, clearing the excess sediment and reshaping the channel.
     • Thus, internal factors within the system control when erosion or deposition occurs.
   • Complex Response
     • When a river system experiences rejuvenation (due to tectonic uplift or climatic changes), its response is not immediate or uniform across the entire system.
     • Different sections of the river adjust at different rates, leading to multiple simultaneous processes like:
       • Downcutting (deepening of valleys)
       • Aggradation (sediment buildup in some areas)
       • Renewed erosion in other sections
     • This makes landscape evolution more complex than a simple cycle of erosion.

The Episodic Nature of Erosion & Deposition

🔹 Schumm argued that landscapes do not evolve in a linear or progressive manner but experience irregular cycles of change.

🔹 This means that the classic “cycle of erosion” (youth → maturity → old age) does not always apply because periods of erosion and stability occur repeatedly.

🔹 If an external force like isostatic uplift interacts with geomorphic thresholds and complex response mechanisms, the result is an even more complex geomorphic cycle with alternating stable and unstable periods.

Why Is Landscape Evolution So Complex?

📌 Key Insight: Changes in one part of a river system do not instantly affect the entire river.

🔸 If sediment builds up in one section, it takes time for the effects to propagate downstream.
🔸 Some areas may experience erosion, while others remain stable, leading to multiple landform adjustments occurring simultaneously.
🔸 This creates a landscape with varied and unpredictable features, rather than a simple, uniform progression.

Comparison of Schumm’s Model with Other Theories

Criticism of Schumm’s Model

🔹 Difficult to measure geomorphic thresholds in real-world settings – How do we precisely determine when a landscape reaches a critical threshold?
🔹 Does not fully explain long-term landscape stability – Some landscapes appear stable for millions of years, while Schumm’s model suggests frequent disruptions.
🔹 External forces like climate change and tectonics still play a major role – While Schumm focused on internal factors, external events can still trigger significant landscape changes.