Lawn depressions manifest as sinking areas or an uneven grade across an otherwise flat surface. These irregularities detract from a lawn’s aesthetic appearance, pose tripping hazards, and complicate routine maintenance tasks like mowing. Identifying the specific underlying cause of these uneven spots is necessary, as the repair method must directly address the root problem, not just the surface symptom. Depressions typically involve changes or disturbances beneath the soil surface over time. Causes range from the biological decay of buried materials to the physical movement of soil due to water or human activity.
Decomposition of Buried Organic Matter
One of the most frequent causes of localized depressions, especially in older or recently developed properties, is the breakdown of large organic materials beneath the sod. When land is cleared for construction, roots, stumps, or substantial pieces of wood are sometimes buried rather than fully removed. This organic mass, often deep below the topsoil, begins a process of slow, anaerobic decomposition.
Microorganisms gradually consume the buried material, converting the solid wood structure into gases, water, and residual humus. As the material’s bulk is reduced through this decay process, the volume it occupies underground decreases significantly. The soil column directly above the shrinking mass loses its support, creating a subsurface void. This void remains stable until the weight of the overlying soil, combined with heavy rain or foot traffic, causes the soil crust to collapse. This collapse results in a sudden or gradual depression that may not appear until several years after construction or tree removal has occurred.
Subsurface Water Movement and Soil Settling
Physical disturbances and the dynamics of water within the soil profile represent the most common causes of lawn unevenness. A primary culprit involves the uneven compaction of backfilled soil, particularly above trenches dug for utility lines such as water, sewer, or electrical conduits. When soil is excavated and then returned to a trench, it is often not re-compacted to the same density as the surrounding, undisturbed native soil.
The backfilled soil, especially if it contains large air pockets, will continue to settle over months or even years. This differential settling results in linear depressions that precisely trace the path of the underlying utility line, creating a distinct sunken trough across the lawn. This effect is compounded in areas with high clay content, where the soil structure is more prone to volume change as moisture levels fluctuate.
Water movement from sources other than surface runoff can also erode the subsurface. Underground leaks from damaged irrigation lines, sprinkler heads, or broken plumbing introduce a constant flow of water directly into the soil. This steady saturation washes away the finer soil particles, or “fines,” from the subsoil structure, leaving behind larger, less cohesive material and creating a hollow space beneath the surface.
As the underlying fine soil is transported away, the remaining structure becomes unstable, eventually leading to a depression or even a small sinkhole at the surface. Similarly, severe surface runoff, particularly on sloped yards, can channel water into small cracks or existing low points, eroding the soil base and causing the low spots to deepen over time. This erosion is directly tied to poor surface grading that fails to direct water away from vulnerable areas.
Pest and Wildlife Burrowing
Biological activity in the form of burrowing animals can cause localized, sporadic depressions distinct from settling issues. Common yard pests like moles, voles, and gophers create extensive underground tunnel networks, which compromise the stability of the immediate subsoil. Moles, which are insectivores, tunnel just beneath the surface, creating shallow, raised ridges or “runways” that feel spongy when stepped on.
These shallow tunnels are temporary voids whose roofs are easily fractured by surface pressure, resulting in small, linear collapses. Gophers, conversely, are rodents that excavate deeper, more extensive tunnel systems. When a gopher abandons a section of its network or when heavy moisture saturates the soil above a tunnel, the tunnel’s roof can collapse, leaving a sudden, localized depression.
Voles, which are smaller rodents, primarily create surface runways, but some species also create shallow tunnels just below the surface. Regardless of the specific animal, the mechanism of depression formation is the creation of a hollow space that eventually collapses under the weight of the overburdened soil, causing a visible dip in the lawn.