中文摘要：

珠江三角洲表土的粒度组成以粉砂为主，床沙则以混合砂为主，北江的床沙比西江的粗。20世纪90年代与70年代对比，床沙的粒度组成变细；中值粒径妒值变大；运动方式的推移组分增大。床沙的细化率西江（21．9％）比北江（15．8％）大。按河流段、近口段、河口段的划分，细化率的沿程变化，西江为从上游向下游增大，北江则相反。床沙细化的主要原因是河道纵比降变小，纵比降变小是河道采沙及分流比变化所造成的。

英文摘要：

The economy developed at a great speed in the Pearl River Delta, where the GDP by 2001 was 29.6 times that by 1987. The annual amount of sand resource used for different construction was 61 × 10^6 -68 × 10^6m^3 on average. During the period of 1984 - 1999 the used sand totaled 915 × 10^6 - 1020 × 10^6m%3 and the total amount of quarried sand from river channels was 700 × 10^6 - 1000 × 10^6m^3. As a great deal of coarse sand was quarried from river channels, the fine sand was deposited in the riverbed. However, the main reasons why the riverbed sand became fine were that quarrying sand resulted in the drop of the riverbed, and the increase of the flow ratio of the North River strengthened the hydro-dynamics and the downcutting of the riverbed. As a result, the gradient of river channel decreased, the fine-grained sediments deposited easily. The component of fine grain of the riverbed sand increased from 65.6% to 73.4% in the West River during the period of 1977 - 1998, and from 55.2% to 59.0% in the North River during the period of 1979 - 1998. The Ф value of medium grain in 1998 was larger than that in 1976. The fineness ratio in the West River was 21.9%, while that in North River 15.8%. The fineness ratio varied at different sections. For example, the high value of fineness occurred at upper section of the North River, while that at lower section of the West River. That was corresponding essentially to the distribution of the scoured channels, showing the decrease of the gradient of river channels caused by scouring.