Exam #3

Ocean/lakes/soil → evaporation + plant transpiration → atmosphere/clouds → precipitation → (1) runoff to streams/rivers/ocean, (2) infiltration → groundwater recharge → groundwater discharge to streams/ocean, and (3) snow/ice storage → meltwater runoff.

Drainage networks = connected systems of channels that collect runoff and groundwater discharge and route water downslope toward a common outlet. They form as flowing water erodes weak pathways, lengthens channels upstream by headward erosion, and integrates tributaries into branching patterns controlled by slope, bedrock, structure, climate, vegetation, and base level.

A drainage basin, or watershed, is the land area where precipitation, runoff, and groundwater flow drain toward a shared outlet, such as a stream, lake, estuary, or ocean. Its boundaries are defined by topography and sometimes groundwater divides. A drainage divide is the high ground, ridge, or subtle topographic boundary that separates one drainage basin from another. Water falling on one side of a divide flows toward one outlet, while water falling on the other side flows toward a different outlet.

To delineate a drainage basin on a topographic map, first identify the outlet point of the stream or river. Then trace upstream along all tributaries that feed that outlet. Next, use contour lines to find the surrounding high points and ridgelines that separate flow toward the outlet from flow toward neighboring valleys. Draw the basin boundary perpendicular to contour lines where it crosses slopes, connecting hilltops and ridges around the drainage network until the line returns to the outlet. The finished boundary should enclose all land that would drain to the selected outlet and exclude land draining to other streams.

Area * veloicty Stream discharge is the volume of water passing a cross-section of a channel per unit time, commonly reported in cubic meters per second or cubic feet per second. It equals channel cross-sectional area multiplied by average flow velocity. Discharge is controlled by precipitation amount and intensity, drainage-basin size, slope, soil and rock permeability, vegetation, snowmelt, groundwater input, evapotranspiration, antecedent soil moisture, land use, dams, and channel shape. Discharge usually increases downstream because tributaries and groundwater add water, though it can decrease in arid regions or losing reaches where water infiltrates into the ground.

Q = velocity * cross-section area

Streams erode landscapes by hydraulic action, abrasion, solution, and undercutting of banks and bedrock. In steep headwaters, erosion is mostly vertical, producing narrow valleys, waterfalls, rapids, and V-shaped valleys. As gradient decreases downstream, lateral erosion becomes more important, widening valleys and creating meanders. Streams deposit sediment when velocity and transport capacity decline, forming point bars, floodplains, natural levees, alluvial fans, braided bars, deltas, and terraces. The balance between erosion and deposition depends on discharge, slope, sediment load, grain size, vegetation, base level, and changes in climate or land use.

Meanders form because flow velocity is not uniform across a bend. Water flows fastest along the outside of a bend, where it erodes the cut bank, and slowest along the inside of a bend, where sediment is deposited on a point bar. This causes bends to grow wider and migrate downstream over time. Adjacent meanders can eventually narrow the neck between bends, and during a flood the river may cut through the neck, leaving behind an abandoned loop called an oxbow lake. Meanders develop best on low-gradient floodplains where banks are erodible and lateral erosion dominates over vertical incision.

As velocity decreases, the river can no longer transport its full sediment load, so sediment is deposited near the mouth. Coarser sediment settles first and finer sediment is carried farther offshore. Over time, repeated deposition builds outward into the receiving basin, creating distributary channels, mouth bars, wetlands, and delta lobes. Delta shape depends on the balance among river sediment supply, waves, tides, sea-level change, subsidence, and human modifications such as dams or levees.

Downstream, channel width and depth generally increase because tributaries and groundwater add water. Discharge usually increases downstream for the same reason. Average flow velocity often increases slightly downstream despite lower gradient because channels become smoother, deeper, and more efficient. Maximum grain size usually decreases downstream because large particles are deposited upstream and sediment becomes rounded and broken down during transport. Total sediment load commonly increases downstream as more tributaries contribute sediment, but the average particle size becomes finer. The proportion of dissolved and suspended load often increases relative to bed load.

Flooding occurs when water covers land that is normally dry, often because stream discharge exceeds channel capacity. Causes include intense rainfall, long-duration storms, rapid snowmelt, hurricanes, storm surge, ice jams, dam or levee failures, saturated soils, low infiltration, and urbanization. Flood damage can be reduced through floodplain zoning, avoiding construction in high-risk areas, restoring wetlands and riparian buffers, maintaining natural floodplains, improving stormwater systems, using retention/detention basins, elevating structures, floodproofing buildings, early-warning systems, evacuation planning, and carefully designed levees or floodwalls. Structural defenses can reduce some risks but may also create a false sense of security and worsen downstream flooding if floodplains are disconnected from rivers.

Flood probability is usually calculated from a ranked record of annual peak discharges. The largest flood in the record is ranked 1, the second largest is ranked 2, and so on. A common recurrence interval formula is RI = (n + 1) / m, where n is the number of years of data and m is the rank of the flood. Annual exceedance probability is then P = 1 / RI, often expressed as a percentage. For example, a 100-year flood has a 1/100 = 1% chance of being equaled or exceeded in any given year. This does not mean it happens exactly once every 100 years; it means each year has an independent 1% probability, assuming climate and watershed conditions are stationary.

First rank the annual peak flows from largest to smallest. Then calculate recurrence interval with RI = (n + 1) / m, where n is the total number of years and m is the rank of the flood of interest. Annual flood probability is P = 1 / RI. Example: if a flood is the 4th largest in a 39-year record, RI = (39 + 1) / 4 = 10 years, so P = 1/10 = 0.10, or 10% chance of being equaled or exceeded in any year.

A hydrograph plots stream discharge on the y-axis against time on the x-axis. Before a storm, discharge is mostly baseflow, which is sustained by groundwater input. After rainfall begins, discharge rises during the rising limb as runoff, throughflow, and quick groundwater response enter the channel. The highest discharge is the peak flow. The delay between maximum rainfall and peak discharge is lag time. After the storm, discharge declines along the falling or recession limb as water drains out of the basin and the stream returns toward baseflow. A flashy hydrograph has a steep rising limb, high peak, and short lag time, often associated with urban surfaces, steep basins, saturated soils, or intense rainfall. A subdued hydrograph has a lower peak and longer lag time, often associated with permeable soils, vegetation, wetlands, and groundwater storage.

Urbanization replaces permeable soil and vegetation with impervious surfaces such as roads, roofs, parking lots, and sidewalks. This reduces infiltration and groundwater recharge while increasing surface runoff. Storm drains and channelized streams move water rapidly into waterways, which increases peak discharge and shortens lag time after rainfall. Baseflow may decrease over time because less water infiltrates to recharge groundwater, while flood peaks become higher and more frequent. Urban streams often become flashier, more erosive, warmer, more polluted, and more disconnected from natural floodplains.

A warming climate can increase flooding because warmer air can hold more water vapor, which can intensify heavy precipitation events. Warming can also alter snowmelt timing, increase rain-on-snow events, strengthen some tropical cyclones, and raise sea level, which worsens coastal and tidal flooding. Urbanization increases flooding by creating impervious surfaces that reduce infiltration and speed runoff into streams. When heavier rainfall falls on urbanized watersheds, hydrographs become especially flashy, with shorter lag times and higher peak discharges. Together, climate-driven rainfall changes and land-use changes increase the chance that stream discharge will exceed channel or drainage-system capacity.

Groundwater is water stored and moving beneath Earth’s surface in the pores, fractures, and openings of soil, sediment, and rock. It comes mainly from precipitation and surface water that infiltrate into the ground and percolate downward to the saturated zone. Groundwater occurs below the water table, where pore spaces are filled with water. Its flow is controlled by hydraulic gradient, permeability, porosity, rock fractures, aquifer geometry, recharge and discharge rates, pumping, and barriers such as aquitards. Groundwater generally flows from areas of higher hydraulic head to areas of lower hydraulic head, often from recharge areas toward streams, springs, wetlands, wells, or the ocean.

A side-view sketch should show the land surface at the top, soil and sediment beneath it, and a curved water table below the surface. Above the water table is the unsaturated zone, where pores contain both air and water. At the water table, pore spaces become fully saturated. Below the water table is the saturated zone, where groundwater fills connected pore spaces and fractures. A simple text sketch is: land surface → unsaturated zone with air + water in pores → water table boundary → saturated zone with pores filled by groundwater. The water table often slopes toward streams, lakes, wetlands, or the ocean.

Porosity = % of rock that is open space (ability to soak in) Permeability = ability of a material to transmit water through connected openings (ability to move) A material can have high porosity but low permeability if pores are poorly connected, such as in clay. Sand and gravel often have both good porosity and high permeability, making them productive aquifer materials. Crystalline rock may have low primary porosity but can transmit water through fractures.

Aquifer = permeable rock or sediment layer that can store and transmit usable quantities of groundwater Aquitard = boundary, low-permeability layer, such as clay or shale, that slows groundwater flow and can confine an aquifer Water table = upper surface of the saturated zone in an unconfined aquifer. Water-table depth changes with the balance between recharge and discharge. Climate, season, topography, soil permeability, vegetation, and land use also affect water-table depth.

A gaining stream receives groundwater because the water table is higher than the stream channel, so groundwater flows into the stream. Gaining streams are common in humid regions, valleys, and places where groundwater discharge sustains baseflow. A losing stream loses water to the subsurface because the water table is below the stream channel, so stream water infiltrates downward and recharges groundwater. Losing streams are common in arid regions, alluvial fans, areas with heavy pumping, and places where channels cross permeable sediment above a deep water table. Some streams can switch between gaining and losing depending on season, flood stage, drought, or groundwater withdrawal.

Connect the dots from the river elevation to the groundwater -> continue on the other side -> forms V’s -> draw groundwater flow perpendicular *losing = v points downstream gaining = v points upstream

An unconfined aquifer has its upper boundary at the water table and is directly recharged from the surface by infiltration. Its water level rises and falls with recharge, drought, and pumping. A confined aquifer is overlain by a low-permeability aquitard that separates it from direct surface recharge. Water in a confined aquifer is under pressure, so in a well it rises above the top of the aquifer to a potentiometric surface. If that pressure surface is above land surface, the well may flow naturally as an artesian well. Confined aquifers are often recharged where the aquifer layer is exposed at the surface or connected to recharge zones upslope.

Karst landscapes form where slightly acidic groundwater dissolves soluble rock, especially limestone, dolostone, or gypsum. Rainwater absorbs carbon dioxide from the atmosphere and soil, forming weak carbonic acid. As this water moves through fractures and bedding planes, it dissolves carbonate minerals and gradually enlarges openings into conduits, caves, and underground drainage networks. Surface features include sinkholes, disappearing streams, springs, solution valleys, and tower karst. Caves commonly form near or below the water table where groundwater flow is concentrated. If the water table later drops, formerly flooded passages may become air-filled caves with speleothems such as stalactites and stalagmites.