The Northern Piedmont in Delaware is represented by one Level IV Ecoregion, the Piedmont Uplands (64c). This ecoregion is characterized by rounded hills, low ridges, relative high relief, and narrow valleys and is underlain by metamorphic rock. The dominant historical vegetation was oak and oak-hickory forest, with a lesser extent of mixed mesophytic forest. An important ecological feature of the ecoregion is the occurrence of scattered serpentine barrens that support specialized flora and fauna. The boundary of the Piedmont Uplands follows the limit of ancient metamorphic rock, distinct from the largely sedimentary rock of the surrounding ecoregions.  

The Southeastern Plain Ecoregion (65) is represented in Delaware by a small area of the Chesapeake Rolling Coastal Plain (65n) Level IV Ecoregion, a hilly upland with narrow stream divides, incised streams, and well-drained loamy soils. It is hillier and better drained than the Middle Atlantic Coastal Plain (63) Ecoregion (see below), with older sedimentary rocks. Stream channels are relatively low in gradient and are often swampy-margined and sandy-bottomed. The most common soils are low-nutrient Ultisols that support oak-hickory-pine forests.

The majority of the state falls within the Middle Atlantic Coastal Plain (Ecoregion 63), which consists of three Level IV ecoregions, as detailed below. 

Delaware River Terraces and Uplands

The areas adjacent to the Delaware River and Bay, the Delaware River Terraces and Uplands (63a), are narrow, marshy, nearly level to rolling lowlands dominated by tidal marshes and meandering, low gradient streams, which are often tidally influenced. Saline marsh deposits and alluvial and estuarine sand and silt are underlain by unconsolidated and easily eroded Quaternary gravels, sands, and silts.  

Barrier Islands – Coastal Marshes 

The Barrier Islands-Coastal Marshes Ecoregion (63d) is composed of beaches, dunes, low terraces, beach ridges, and barrier islands that are fringed by lagoons, bays, tidal salt marshes, mudflats, tidal channels, or ocean. The vegetation is mostly salt marsh, which contrasts with the natural hardwood vegetation of the Delmarva Uplands (63f). Oak-Hickory-Pine Forest occurs in better drained, higher areas. The western boundary with the Delmarva Uplands (63f) generally follows a long, often poorly defined, east-facing scarp that parallels the present shoreline at about 20 feet (6 m) above sea level. The presence of this scarp is an impediment to inland migration of tidal marshes in response to sea level rise.  

Delmarva Uplands  

The Delmarva Uplands (63f) Ecoregion include sandy ridges, swales, and the central ridge of the peninsula. Marshes and swamps are far less extensive than in 63a and 63d above, but do occur and include the Great Cypress Swamp of southern Delaware. Many wet, shallow elliptical depressions (Delmarva Bays) occur in this Ecoregion.  

Parsonsburg Sand covers broad areas; its surface consists of sinuous, low sand ridges and broad, seasonally-wet, swales (Denny et. al., 1979). Ultisols are common, supporting a natural vegetation of mostly oak-hickory pine forest. Sandy soils are nutrient poor and have a limited water holding capacity   

Streams and rivers are low gradient, often tidally influenced, and have wide valleys. Many have been straightened and deepened to improve drainage. Streams on the well-drained uplands have riffle sections with gravelly bottoms. 

Delaware is in a transition zone between humid subtropical climate conditions to the south and humid continental conditions to the north. The moderating effects of the Chesapeake and Delaware Bays and the state’s proximity to the Atlantic Ocean lessen temperature extremes compared to nearby interior locations. Even so, the state has a continental climate, with cold winter temperatures, hot summers, and ample precipitation throughout the year (Leathers 2015). 

Mean annual temperatures across the state range from 54.0 °F in northern New Castle County to 58.1 °F along the Atlantic coast of southern Delaware. Average annual precipitation is approximately 45” statewide (Leathers 2015). 

The State is often affected by seasonally occurring severe weather including winter and spring nor’easters that can drop heavy snow and cause coastal flooding, autumn tropical systems with high winds, coastal flooding and heavy rainfall, and spring and summer severe thunderstorms. 

Historical Trends 

Observed historical data indicate that temperatures across Delaware have been increasing since 1895, with a noticeable increase in mean annual temperatures since about 1980. This warming trend includes all seasons and is asymmetrical, with greater increases in minimum temperatures, especially in more recent years, than in maximum temperatures. There have also been increases in the frequency of warm temperature extremes, and decreases in the frequency of cold temperature extremes. Statewide precipitation has shown no significant changes since 1895, except for a significant upward increasing trend during the autumn season.  

To better understand the current and future vulnerabilities and risks of climate change, Delaware Department of Natural Resources and Environmental Control (DNREC)’s Division of Energy and Climate conducted a statewide climate change impact assessment (DNREC 2014). The Delaware Climate Change Impact Assessment reflects the best available climate science, climate modeling, and projections to illustrate the range of potential vulnerabilities that Delaware may face from the impacts of climate change.