Stream burial—the routing of streams through culverts, pipes, and concrete lined channels, or simply paving them over—is common in urbanized areas [1
]. In many regions, the majority of buried reaches are headwater (1st and 2nd order) streams, including ephemeral and intermittent reaches, with removal of up to 70% of headwater stream length in some areas [1
]. While the cumulative effects to ecosystem processes wrought by stream burial have important ramifications for regulation under the U.S. Clean Water Act [6
], the extent of burial has not been assessed for most urban areas. Knowledge of the extent of burial events is critical for effective resource management, including preservation of remaining intact streams, restoration of urban streams, and assessing cumulative impacts of urbanization to water quantity and quality.
As with other forms of stream modification, burial alters the primary physical, chemical, and biological processes in headwater systems, contributing to a state of degradation commonly referred to as “urban stream syndrome” [7
]. With the percentage of the world’s population living in urban areas continuing to grow [8
], a greater emphasis has been placed on understanding the structure and function of urban streams, and associated impacts to human health, and that of downstream ecosystems. The identification of stream burial as a critical and pervasive driver of the “urban stream syndrome” [7
], and the recognition that key questions remain in regards to ecosystem structure and function within piped ecosystems [9
], has led to increased research activity, and the explicit consideration of stream burial in formulation of long-term research agendas (e.g., the Baltimore Long Term Ecological Research (LTER) site; Sujay Kaushal, personal communication
). Subsequent attention has focused on extensive stream burial as a key contributor to “urban stream deserts” [4
], riverless urban areas that evolved primarily due to stream burial for human development and population growth [10
], and the phenomenon of convergent surface water distributions across urban areas of the United States [11
], whereby there is a decrease in variance in the density of surface flow channels—streams and rivers—with increasing development intensity, largely due to burial of stream channels.
While not extensive to date, studies of stream burial have demonstrated a consistent set of interrelated ecosystem impacts, including modified flow velocities, altered carbon and nutrient inputs, and amplified nitrogen transport [9
], loss of habitat and decreased nutrient subsidies [12
], and barriers to dispersal of aquatic organisms [13
]. More recently, Kaushal and Belt [14
] have recognized stream burial as part of an “urban stream continuum”, whereby extensive engineering of headwater systems has expanded natural flowpaths (“urban karst”), leading to increased hydrologic connectivity within watersheds, thereby influencing the flux and transformation of nutrients, contaminants, and energy across both space and time. Leveraging this novel conceptual framework, subsequent research has focused largely on biogeochemical cycles within buried headwater streams, documenting significant reductions in nitrogen (N) uptake, gross primary production (GPP), and ecosystem metabolism (ER), with potential to influence watershed nutrient exports to downstream waters [15
Research has demonstrated a consistent negative effect of increasing levels of urbanization on various indicators of stream health [7
]. Most studies have relied on total imperviousness (TI; the proportion of a watershed that is covered in impervious surface) as the primary measure of urbanization impacts on freshwater ecosystems as TI is viewed as an integrative and comprehensive indicator [20
] that can be readily incorporated into land use planning [12
]. However, impervious cover alone has proven an insufficiently sensitive measure of river health [23
], as significant aquatic assemblage degradation has been observed across a wide range of watershed imperviousness [24
Recognition that the spatial configuration of impervious cover relative to stream channels may be an important moderator of the magnitude of stream ecosystem response to urbanization [25
] has led to development of alternative metrics for measuring urbanization effects on stream ecosystems. Measures of effective imperviousness (EI, impervious cover directly adjacent to a stream channel [27
]) and directly connected impervious area (DCIA, impervious surfaces that route stormwater runoff directly to streams via stormwater pipes [29
]) have been shown to better integrate the multiple stressors of urban development, relative to TI. However, these methods also have shortcomings; they either fail to explicitly capture piped and concrete-lined stream channels (e.g., EI) or necessitate detailed information on stormwater conveyances to determine runoff routing and specific on-lot drainage patterns (DCIA). Most importantly, neither approach directly quantifies the impact of urbanization on stream habitat, and instead relies on indirect measures such as changes in sedimentation and hydrology.
Stream habitat is most directly impacted when impervious surface completely covers the stream channel. Previous research has shown a relationship between stream size and the probability of burial, with the smallest, headwater streams of the urbanized Gunpowder-Patapsco watershed, Maryland, USA, exhibiting disproportionately high rates of burial in relation to larger streams [2
]. Whether this pattern remains consistent in other watersheds or across broader geographic scales is unknown. Local topographic patterns, such as slope, are also known to affect the probability of urbanization [30
], by making some places inaccessible or unstable for building [32
]. It remains unclear whether these same physiographic constraints may limit, or necessitate, the burial of streams. Insights into both the spatial and temporal patterns of stream burial, particularly with respect to stream size and topographic slope, are critical for gauging the effectiveness of land-use policies meant to foster development, while protecting the health of stream ecosystems. Historical patterns of stream burial also provide insight into the characteristics of streams that remain, information that is potentially useful for describing and understanding patterns of watershed exports, ecosystem functions, and remaining aquatic biodiversity [33
Recent advancements in stream mapping, remote-sensing of impervious cover, and predictive models now make it possible to predict stream burial at a relatively high level of detail and accuracy across large areas. To enhance our understanding of the phenomenon of stream burial, we developed a novel analytical approach using improved headwater stream maps [35
], moderate resolution impervious cover data [36
], and recursive partitioning models [37
] to map the extent and magnitude of burial across an urban gradient in the Mid-Atlantic United States. We then analyze the spatial patterns of stream burial within the context of watershed area, topography, and impervious surface area. We expect these new burial maps and analysis of burial response to physiographic parameters to provide a fresh perspective into land use decision-making processes, and the development pressures facing critical headwater stream ecosystems, past, present, and future.
Stream burial is a spatially pervasive phenomenon, with predicted burial increasing linearly with total impervious cover across all levels of development, bringing into question the efficacy of existing stream protections. The close relationship between stream burial and total impervious surface area suggests that the two measures provide similar, and perhaps redundant, information. However, stream burial maps provide a spatially-explicit measure of potential stream-specific impacts, accounting for the effects of impervious surface area immediately adjacent to and covering stream channels, including direct habitat loss, and the probable effects of contiguous impervious cover on physical and hydrologic regimes in stream ecosystems. Predicted stream burial data could be used to identify high impact watersheds for targeting restoration, to address riparian and network connectivity issues, and to integrate effects of hydrological change into efforts to manage downstream water quality, including the Federally-regulated TMDL (Total Maximum Daily Load) process.
We know that loss of in-stream and riparian habitat holds potential implications for aquatic organisms, and their ability to move both within and between headwater systems. Future work should consider the effects of stream burial on network geometry (the size and spatial orientation of remaining stream reaches), and the effects on habitat connectivity within and between headwater systems on biodiversity patterns in aquatic communities. Future work might also include examining predicted burial rates across time, to better discern how burial has proceeded in relation to physiographic and policy constraints, and related effects to ecosystem structure and function across large, developing watersheds.
Headwater stream burial is prevalent across the study area, even within watersheds with very little urban development. Both slope and catchment area combine to limit stream burial during development, but these constraints were largely overcome in the most intensely urbanized jurisdictions. Headwater stream systems are critical to the maintenance of downstream water quality and hydrologic regimes [53
], and yet, continue to be disproportionately affected relative to larger streams. This understanding might be used to justify more rigorous and uniform protection policies and other strategies to reduce the impacts of burial and preserve the ecological function of these vital ecosystems.